fix(rust): route messages to current session owner

[gimenete]

Summary

• Keep per-session JSON-RPC request and session.event enqueue atomic with router registration lookup, preventing delivery through a stale sender after a session ID is replaced.
• Preserve session-ID reuse, unknown-session dropping, and the existing wire protocol; no child-process lifecycle changes were needed.

Fixes #2791

Validation

• cargo test --no-default-features --features test-support --lib router::tests
• cargo clippy --no-default-features --features test-support --lib -- -D warnings
• cargo +nightly-2026-04-14 fmt --all -- --config-path .rustfmt.nightly.toml --check
• git diff --check

[gimenete]

Closing after further review: this is not an observable bug. The router task is the sole producer on each per-session channel and unbounded sends never block, so sending through a sender cloned before a concurrent re-registration is indistinguishable from linearizing the send at lookup time (before the replacement). Holding the lock across enqueue changes no reachable outcome, and the added test passes against the pre-change semantics, so it is not a regression test. No change needed.

[Copilot]

Copilot review overview

🟡 Changes recommended

The test does not reproduce the concurrency race the change is intended to prevent.

Review effort: Balanced
Findings: 1

Open (1)

• Add regression test for sender replacement race · New

What changed in this PR

This PR fixes stale Rust session routing by making sender lookup and message enqueue atomic.

Changes:

• Routes requests and notifications while holding the session registry lock.
• Adds coverage for reused session IDs.

File	Description
rust/​src/​router.rs	Adds atomic routing helpers and related tests.

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