## Motivation and Context
Third step of the stateless lifecycle (SEP-2575, modelcontextprotocol/modelcontextprotocol#2575) for
the 2026-07-28 MCP spec release. `StdioTransport` now serves the legacy handshake lifecycle
and the modern per-request-envelope lifecycle on one connection, with the same era-lock semantics as
the TypeScript and Python SDKs: the first era-distinctive message to SUCCEED locks the connection era
for its lifetime.
- Each stdin frame is parsed once (`symbolize_names: true`) so era classification can inspect its method
and `_meta`. Frames that are not JSON objects fall back to `ServerSession#handle_json`,
keeping protocol-level error responses byte-identical.
- A successful `initialize` locks `:legacy` (already a side effect of `ServerSession#mark_initialized!`).
A successful `server/discover` or a successful request carrying the full modern `_meta` triple locks `:modern`.
Failed era-distinctive messages (for example an unsupported envelope version) leave the connection unlocked,
so a client probe can still fall back to the other era.
- Era violations are rejected in-band by `Server#lift_request_envelope`, which now also covers the legacy side:
a modern envelope arriving on a legacy-locked session is an invalid request (`-32600`), mirroring
the existing modern-side rules (`initialize` after a modern lock is `-32022`; a missing envelope after
a modern lock is `-32600`).
- `StdioTransport#send_request` raises on a modern-locked session: the modern lifecycle forbids
server-initiated JSON-RPC requests, which multi round-trip `input_required` results (SEP-2322) replace.
The inline read loop inside `send_request` dispatches through the same era-aware path as the main loop.
Refs #389.
## How Has This Been Tested?
New tests in `test/mcp/server/transports/stdio_transport_test.rb` drive full stdin/stdout round trips:
legacy lock via `initialize` then rejection of a modern envelope, modern lock via `server/discover`
then `-32022` (with `data.supported`) for a late `initialize`, modern lock via an envelope-carrying request,
no lock when the probe fails with an unsupported version (a legacy `initialize` still succeeds afterwards),
the envelope requirement after a modern lock, and the `send_request` prohibition on modern sessions.
`bundle exec rake` (tests, RuboCop, and conformance baseline, including the stdio conformance scenarios) passes.
## Breaking Changes
None. Legacy clients send `initialize` first and take the same code path as before;
the era lock only constrains message sequences that mix lifecycles on one connection,
which no legacy client produces.
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