[Schema][Server] Close the 2025-11-25 schema gaps and add the 2026-07-28 surface (#421)

* [Schema][Server] Close the 2025-11-25 schema gaps and add the 2026-07-28 surface

Ports the type definitions this SDK still misses outside of sampling tool use,
which #409 and #420 already cover. Every addition is optional and defaults to
current behaviour, so a connection negotiated on an older revision is
unaffected.

From 2025-11-25, elicitation gains modes. ElicitationMode splits `form` —
build a form from the requested schema — from `url`, which sends the user out
of band and returns only the accept/decline/cancel outcome. That is why
requestedSchema becomes optional and `url` appears beside it.
ClientCapabilities learns the matching sub-capabilities, where an
`elicitation` naming no mode declares form, the only shape that existed
before. Icon gains `theme` from the same revision, and Implementation gains
the `title` BaseMetadata has carried since 2025-06-18.

From 2026-07-28, schemas loosen where the revision loosens them: SEP-2106
drops the object-only restriction, so Tool::outputSchema may describe any JSON
value and CallToolResult::structuredContent follows.

The same revision defines three error codes (-32020 header mismatch, -32021
missing required client capability, -32022 unsupported protocol version).
ProtocolVersionMiddleware switches to the last of them, so a rejected version
carries the supported set as structured data the client can retry from rather
than only as prose.

* [Schema] Reject what the 2026-07-28 types cannot represent

`Implementation::title` reached the typed constructor unchecked, so malformed
wire data raised a TypeError instead of InvalidArgumentException.

`ToolUseContent::input` accepted a list and serialized it as a JSON array,
where the protocol requires an object. The empty array stays exempt: it is
also an empty map and still emits `{}`.

`ToolChoice` and `ElicitRequest` read their mode with isset(), which is false
for an explicit null, so `{"mode": null}` silently became the default instead
of being rejected. Both use array_key_exists() now, letting the existing type
check refuse null.

* [Client][Server] Advertise the implementation title through both builders

`Implementation::title` could be parsed but never sent: neither
`Client\Builder::setClientInfo()` nor `Server\Builder::setServerInfo()`
accepted one, so every SDK user emitted null.

Both gain a trailing optional `$title`. On the server it sits where the
Implementation constructor already puts it, so existing positional calls keep
their meaning; the client builder forwards it by name, leaving the icons and
websiteUrl slots defaulted.

* [Schema] Serialize every non-null structuredContent

The object-only hydration guard was never that: `!is_array()` admitted
`[1, 2, 3]` and `[]`, which serialize to JSON arrays, while rejecting the
scalars 2026-07-28 permits. The truthiness emission gate was backwards in the
same way — it dropped `[]`, `0`, `false` and `""`, yet emitted lists, strings
and an empty stdClass.

Hydration now accepts any JSON value, and `null` alone means absent, matching
`ToolResultContent` which already carries this field that way. Which values a
given revision permits is a question for version-aware serialization, which
results cannot answer yet.

* [Schema][Server] Make url elicitation reachable

`ElicitRequest::forUrl()` built a request no SDK user could send: the only
public gateway method always constructed form mode from an ElicitationSchema,
and `request()` is private.

`elicitUrl()` joins `elicit()`, and both funnel through one send path that
hydrates the result with the request's own mode. Without that, a url-mode
accept — contentless by design — threw, because ElicitResult requires content
whenever the action is accept. The result carries no discriminator of its own,
so the mode has to come from the request it answers.

`supportsElicitationUrl()` reports whether the client named the mode, reusing
the sub-capability reader the sampling checks already use.

* [Schema] Add CHANGELOG entry for the 2026-07-28 surface

* [Capability][Schema][Server] Carry the widened structuredContent end to end

Review follow-up on the SEP-2106 widening, which stopped at the type.

`ToolReference::extractStructuredContent()` returned `?array`, so a tool
declaring a scalar `outputSchema` had its result dropped and logged as
unsendable. It returns `mixed` now and keeps a scalar — but only from
2026-07-28 on, and only when the tool declared an outputSchema: without one
the value is already carried in `content`, and advertising it twice is not an
improvement.

`CallToolHandler` only warned about list-shaped `structuredContent` on a
self-built `CallToolResult`, so a scalar reached revisions that require an
object unremarked. The check is on the shape now, not on the list case alone.

`ElicitResult` retained a `content` the spec says is absent from url-mode
results, leaving a malformed response indistinguishable from a valid one.

`Tool::jsonSerialize()` emitted the empty root schema as `[]`, which is not a
schema at all. Sub-schemas already had this treatment.

CHANGELOG.md

@@ -16,6 +16,7 @@ All notable changes to `mcp/sdk` will be documented in this file.
 * Add sampling with tools support: sampling requests now accept tools and tool-choice preferences, messages support tool-use/tool-result content blocks and multiple content blocks, and clients can advertise the `sampling.context` and `sampling.tools` capabilities. Adds `ClientGateway::supportsSamplingTools()` / `supportsSamplingContext()` to check the sub-capabilities before sending, and `CreateSamplingMessageRequest::validateToolFlow()`, which asserts the spec's tool-flow rules across the whole message list — the client handler rejects a violating request with `-32602` instead of leaving it unanswered, and the gateway refuses to send one.
 * [BC Break] `SamplingMessage::$content` and `CreateSamplingMessageResult::$content` may now hold a list of content blocks instead of a single one, so code reading them directly must handle both. Use the new `getContentBlocks()` on either class to always get a list.
 * [BC Break] `CreateSamplingMessageResult` now rejects any role other than `assistant`, and rejects empty content, as the specification requires.
+* Close the schema gaps left in `2025-06-18` and `2025-11-25` and add the non-sampling part of `2026-07-28`, all of it optional and defaulting to current behaviour. From `2025-11-25`: url-mode elicitation (`ElicitationMode`, `ElicitRequest::forUrl()`, `ClientGateway::elicitUrl()` and `supportsElicitationUrl()`), whose result carries the user's action alone — `ElicitResult::fromArray()` takes the request's mode, requires content only in form mode and rejects it in url mode; the `elicitation.form` / `elicitation.url` sub-capabilities, where a capability naming no mode declares form; and `Icon::theme`. From `2025-06-18`: `Implementation::title`, now settable through `Client\Builder::setClientInfo()` and `Server\Builder::setServerInfo()`. From `2026-07-28` (SEP-2106): `Tool::outputSchema` and `CallToolResult::structuredContent` accept any JSON value — `ToolReference::extractStructuredContent()` keeps a scalar when the tool declared an outputSchema and the negotiated revision allows it, and `CallToolHandler` warns when a self-built result carries a value the revision does not permit — plus the revision's three error codes (`-32020` header mismatch, `-32021` missing required client capability, `-32022` unsupported protocol version), the last of which `ProtocolVersionMiddleware` returns with the supported set as structured data the client can retry from.
 
 0.7.0
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