mcp: Implement support for methodDiscover on client side

[guglielmo-san]

Implements the client half of the SEP-2575 / 2026-06-30 protocol on top of
part 1 (which added per-request _meta validation on the server). This CL is
client-only; server-side server/discover is still a MethodNotFound stub.

Changes

• server/discover probe in Client.Connect() (client.go). When the
  caller opts into protocolVersion >= 2026-06-30, the client probes the
  server with server/discover before falling back to the legacy initialize
  handshake. On success, the negotiated version is picked from
  DiscoverResult.SupportedVersions ∩ supportedProtocolVersions and stored
  in ClientSession.state.InitializeResult so existing accessors keep working.
• Per-request _meta injection (shared.go, injectRequestMeta). For
  >= 2026-06-30 sessions every outgoing request gets the SEP-2575 triple
  (protocolVersion, clientInfo, clientCapabilities) without overwriting
  caller-set keys.
• DiscoverParams / DiscoverResult types (protocol.go) matching the
  SEP schema, plus methodDiscover registration and the DiscoverRequest
  type alias.
• HTTP transport: setMCPHeaders now falls back to a context-supplied
  protocol version when initializedResult is still nil (needed so the
  discover probe carries the Mcp-Protocol-Version header); the standalone
  GET SSE stream is skipped for >= 2026-06-30 (replaced in a follow-up by
  subscriptions/listen).
• Error plumbing: transport.go wraps closed-connection errors with
  errors.Join so callers can still inspect the underlying jsonrpc2 error
  (used to distinguish UnsupportedProtocolVersionError from
  ErrServerClosing, both -32004).
• Server-side fix: ServerSession.handle no longer pre-populates
  state.InitializeParams from a server/discover request's _meta, so a
  subsequent fallback initialize is not rejected as a duplicate.

Fallback rules

Client falls back to the legacy initialize handshake when discover returns:

• MethodNotFound (-32601),
• UnsupportedProtocolVersionError (-32004),
• an HTTP Bad Request (substring-matched on the error message, to interop
  with pre-2026-06-30 servers that 400 on the unknown _meta.protocolVersion).

Fixes #966