GeezerVison

Fixes #1317

Motivation

Server features (e.g. routing-header checks for methods like tasks/get) need the
decoded Mcp-Name request header, but the modern transport only used it internally
in validateHeaders() and did not expose it through McpTransportContext.

Changes

• New McpNameHeader value, stored under McpNameHeader.TRANSPORT_CONTEXT_KEY,
  modelled on McpParamHeaders.
• HTTP: every validated JSON-RPC request carries a value, attached after the
  user's context extractor so it always wins. The value is one of:
  • ABSENT: the header was not sent;
  • MALFORMED: invalid base64, repeated values, or names differing only in case;
  • the decoded value (isPresent(), value()).
• Stdio: carries the NOT_MIRRORED sentinel, so features can tell "this transport
  has no headers" apart from "the client omitted the header".
• Decoding reuses McpHeaderDecoder and reads all header names/values via the header
  accessor. toString() never includes the value.

Contract: the transport only decodes and forwards. For methods it doesn't validate
itself, the feature owns the equality check against params and should treat
isMalformed() as a mismatch. Existing validateHeaders() behavior is unchanged.

Tests

• McpHeaderDecoderTests: plain, base64, malformed, duplicate, mixed-case names.
• HttpServletMcpTransportIntegrationTests: present / absent / malformed / repeated
  (raw socket for repeated headers), plus regression for existing name/uri validation.
• StdioMcpTransportTests: NOT_MIRRORED sentinel.

Focused run: 111 passed, 0 failed.