Add Stateless Lifecycle Foundations per SEP-2575

## Motivation and Context

SEP-2575 (modelcontextprotocol/modelcontextprotocol#2575, merged for the 2026-07-28 spec release) replaces
the `initialize` handshake with a stateless "modern" lifecycle: every request carries its protocol version,
client info, and client capabilities in reserved `_meta` keys, and the server validates each request independently.
This change lays the shared vocabulary for that lifecycle without changing any behavior:

- `MCP::Configuration` gains `LATEST_MODERN_PROTOCOL_VERSION` ("2026-07-28"), `SUPPORTED_MODERN_PROTOCOL_VERSIONS`,
  and `Configuration.modern_protocol_version?`. Modern versions are deliberately kept out of `SUPPORTED_STABLE_PROTOCOL_VERSIONS`
  so `initialize` can never negotiate them and `protocol_version=` keeps rejecting them.
- `MCP::ErrorCodes` gains `HEADER_MISMATCH` (-32020), completing the spec's stateless lifecycle error code block alongside
   the existing -32021/-32022 constants.
- New `MCP::RequestEnvelope` parses and validates the per-request `_meta` triple (`io.modelcontextprotocol/protocolVersion`,
  `clientInfo`, `clientCapabilities`, plus the optional deprecated `logLevel`). A request is classified as modern only
  when the full triple is present, matching the TypeScript SDK's `RequestMetaEnvelopeSchema` and the Python SDK's `_has_modern_envelope`,
  so partial or legacy `_meta` entries keep flowing through the legacy path untouched.
- New typed errors `Server::UnsupportedProtocolVersionError` (-32022 with `data: { supported:, requested: }`) and
  `Server::MissingRequiredClientCapabilityError` (-32021 with `data: { requiredCapabilities: }`) reuse the established `RequestHandlerError`
  `error_code`/`error_data` plumbing (the -32042 URL elicitation precedent).

Nothing dispatches through `RequestEnvelope` yet; the server-side modern handling, transport routing, and client support build on
this in follow-up changes.

Refs #389.

## How Has This Been Tested?

New `test/mcp/request_envelope_test.rb` covers the modern-triple classification (full triple, partial triple, string keys,
legacy `progressToken`), parsed field access, the -32022 error data shape, and invalid-request errors for incomplete or mistyped triples.
`test/mcp/configuration_test.rb` asserts the new constants, the predicate, and that modern versions stay rejected by `protocol_version=`.
`test/mcp/server_test.rb` asserts both typed errors surface on the wire with their SEP-2575 codes and data through `Server#handle`.
`test/mcp/error_codes_test.rb` pins -32020.

## Breaking Changes

None. Constants and classes are additive; no request path changes behavior.

test/mcp/configuration_test.rb

@@ -66,6 +66,21 @@ module MCP
       assert_equal("protocol_version must be 2025-11-25, 2025-06-18, 2025-03-26, or 2024-11-05", exception.message)
     end
 
+    test "exposes the SEP-2575 modern protocol versions" do
+      assert_equal "2026-07-28", Configuration::LATEST_MODERN_PROTOCOL_VERSION
+      assert_equal ["2026-07-28"], Configuration::SUPPORTED_MODERN_PROTOCOL_VERSIONS
+      assert Configuration.modern_protocol_version?("2026-07-28")
+      refute Configuration.modern_protocol_version?("2025-11-25")
+    end
+
+    test "raises ArgumentError when setting a modern protocol version" do
+      # Modern versions (SEP-2575) are never negotiated via `initialize`, so they are deliberately not settable as
+      # the legacy fallback version.
+      assert_raises(ArgumentError) do
+        Configuration.new(protocol_version: Configuration::LATEST_MODERN_PROTOCOL_VERSION)
+      end
+    end
+
     test "raises ArgumentError when protocol_version is not a boolean value" do
       config = Configuration.new
       exception = assert_raises(ArgumentError) do