Delete the Posture enum, its export, and the posture= constructor
parameter on Server and MCPServer. The lowlevel Server is a registry
of handlers and carries no serving configuration; a server offers both
the 2025 handshake era and the 2026 per-request era on every transport,
and each stream connection's era is decided by the client's opening
message, as the default already did.
_StreamConnection no longer takes a starting era; it starts undecided
and pins on the first era-distinctive message. The streamable-HTTP
manager's header routing is unchanged: streams it has already routed
to the handshake era are entered born-legacy through the transport-
internal serve_legacy_stream, so the legacy leg keeps refusing
enveloped requests with -32600 exactly as before.
The MODERN_ONLY / LEGACY_ONLY tests and doc sections go with it, and
the streamable-HTTP orphan test file is renamed now that its posture
half is gone.
Max Isbey