[Schema][Server] Preserve request id in JSON-RPC error responses instead of fabricating id:"" (#379)

* Preserve request id in JSON-RPC error responses instead of fabricating id:""

When the server received input it could not turn into a valid message, the
error response carried a fabricated empty-string id that was never in the
request, breaking JSON-RPC client correlation.

The id default for the error factories was '' and that default reached the
wire unchanged: unrecoverable parse errors emitted id:"", and for invalid-
but-parseable messages MessageFactory discarded the decoded id so it also
fell back to "".

Now unrecoverable parse errors (-32700) return id: null per JSON-RPC 2.0,
and invalid-but-parseable requests (-32600) preserve the original request
id. Error::$id / getId() and the $id parameter of forParseError() /
forInvalidRequest() are widened to string|int|null, and the recoverable id
is threaded through InvalidInputMessageException.

Fixes #333

* Preserve nullable request id consistently across Error factories and fromArray

- Widen the remaining for*() factories (forMethodNotFound, forInvalidParams,
  forInternalError, forServerError, forResourceNotFound) to string|int|null
  defaulting to null, so no error path can fabricate id:"" again. The only
  defaulting call site (ProtocolVersionMiddleware) now emits id:null instead
  of id:"" for a pre-parse rejection, which is the spec-correct value.
- Accept a null id in Error::fromArray(): require the id key to be present
  (JSON-RPC responses must carry it) but allow its value to be null, so a
  spec-compliant id:null error response round-trips and incoming error
  responses with id:null are accepted instead of rejected.
- Widen the @phpstan-type ErrorData id to string|int|null to match the ctor,
  getId(), and jsonSerialize() shape.

* Document intentional id recovery guard in MessageFactory

* Cover id: 0 and string ids in recoverable-id preservation test

* Fixed client handling of null-id error responses

CHANGELOG.md

@@ -6,6 +6,7 @@ All notable changes to `mcp/sdk` will be documented in this file.
 -----
 
 * [BC Break] `Mcp\Schema\JsonRpc\Error` accepts `null` as its `$id`, and `getId()` may return it. An error response whose id could not be read now omits the member instead of sending `"id": ""` — which claimed the peer had issued a request with an empty-string id. All the `for*()` factories default to `null`, `fromArray()` accepts a missing or explicitly-null id, and `MessageFactory` decodes both as an id-less error rather than rejecting them.
+* Preserve the original request `id` on an invalid-but-parseable message (`-32600`) instead of answering it id-less: `InvalidInputMessageException` now carries the recoverable id via `getRequestId()`/`setRequestId()`, threaded from `MessageFactory` through to the error response.
 * [BC Break] Drop the SDK-only name pattern on `ResourceDefinition`/`ResourceTemplate` `$name` — the spec allows any string (its own examples use `main.rs` and `Project Files`). URI/URI-template validation is unchanged.
 * Add `ClientGateway::supportsExtension()`, `Client\Builder::enableExtension()`, and `ClientCapabilities::withExtensions()` so clients can negotiate and check protocol extensions (e.g. MCP Apps) the same way servers already do. [BC Break] `ServerExtensionInterface` is replaced by the side-agnostic `Mcp\Schema\Extension\ExtensionInterface`.
 * Deprecate Roots, Sampling and Logging per SEP-2577 (protocol revision `2026-07-28`, earliest removal `2027-07-28`). They keep working but using them now triggers a deprecation notice — migrate to tool arguments/resource URIs, a direct LLM provider API, and stderr/OpenTelemetry respectively.