fix(bridge): reject resource subscriptions on rmcp's stateless HTTP path

[bug-ops]

Summary

resources/subscribe/unsubscribe were silently dropped as no-ops when a
request landed on rmcp's stateless per-request HTTP discover-lifecycle path.
rmcp's router (tower.rs) and its request handler (handler/server.rs)
classify a request's lifecycle using different inputs: the router treats a
request as stateless whenever its _meta carries both discover-lifecycle
keys, regardless of what protocol version they declare, while the handler
treats the same request as legacy whenever the declared version predates
2026-07-28. For that mismatch case (_meta present with a pre-2026-07-28
version), the router serves the request through an ephemeral, per-request
McplsServer instance, but the handler dispatches straight to
McplsServer::subscribe/unsubscribe anyway — so the subscription is
recorded into state that's dropped the instant the request completes,
returning result: {} as if it had succeeded.

This closes the gap by rejecting the affected requests with an explicit
error (-32051) instead of a silent no-op. Detection (reject_if_stateless_http
in crates/mcpls-core/src/mcp/server.rs) is gated on the request actually
being HTTP-served (Parts extension present), so stdio — mcpls's primary
transport — is never affected regardless of what _meta a client sends
there. It fires when the request's _meta matches rmcp's own
discover-lifecycle check (via rmcp's public RequestMetaObject::missing_required_keys)
or when the request echoes no Mcp-Session-Id header at all — a present
session header (fabricated, stale, or even from a genuinely live session)
proves nothing on its own, since rmcp never validates it on this branch.

Investigation went through several rounds of adversarial review before
landing on this shape: an initial pass concluded no code change was needed
(rmcp appeared to already reject these requests) but that premise was
falsified by a live repro; a first guard keyed on the session header alone
turned out to be bypassable by attaching any header value; a second version
keyed purely on _meta false-positived on stdio. The final version combines
both signals, gated on the request being HTTP-served.

Changes

• crates/mcpls-core/src/mcp/server.rs — reject_if_stateless_http guard,
  called first in subscribe/unsubscribe; new STATELESS_SUBSCRIPTION_ERROR_CODE (-32051).
• crates/mcpls-core/src/transport.rs — regression tests covering the
  original rmcp-level rejection case, the mcpls-level lifecycle-mismatch
  case (with and without a fabricated session header), the unsubscribe
  case, and non-regression for legitimate legacy sessions.
• crates/mcpls-core/src/bridge/resources.rs — updated SubscriptionRegistry
  doc describing the limitation and how it's addressed.
• CHANGELOG.md — one-line entry under [Unreleased]/Fixed.

Test plan

• cargo +nightly fmt --all -- --check
• cargo clippy --all-targets --all-features --workspace -- -D warnings
• cargo nextest run --workspace --all-features --lib --bins (973 passed, 1 skipped)
• RUSTFLAGS="-D warnings" RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links" cargo doc --no-deps --all-features --workspace
• cargo test --doc --all-features --workspace
• Default (no transport-http) build compiles clean
• New tests independently proven to fail against pre-fix code and pass against the fix

Closes #482