test(bridge): document lock-free translator context

[vcheckk]

Closes #114.

Stacking / readiness

• This PR intentionally remains draft because it depends on fix(bridge): split notification cache lock #180 / issue fix(bridge): split NotificationCache out of Translator lock to eliminate notification_pump starvation #104 and perf(bridge): remove translator mutex from tool requests #181 / issue perf(bridge): reduce Mutex<Translator> hold time — release lock before client.request() #108.
• The test added here documents the lock-free handler context shape introduced by that dependency chain, so it should be reviewed after fix(bridge): split notification cache lock #180 and perf(bridge): remove translator mutex from tool requests #181 land, or rebased once those changes are merged.
• Because I do not have push access to bug-ops/mcpls, I cannot set this PR's base to the fork branch used by perf(bridge): remove translator mutex from tool requests #181 in a way that GitHub will show as an upstream stacked PR.

What changed

• Adds regression coverage documenting that HandlerContext stores Arc<Translator> directly rather than an outer request-wide lock.

Validation

• Branch-local cargo test -p mcpls-core passed.
• Branch-local cargo clippy -p mcpls-core --all-targets -- -D warnings passed.
• Included in standalone integration branch restart-standalone-integration.
• Integration branch cargo test -p mcpls-core passed.
• Integration branch cargo clippy -p mcpls-core --all-targets -- -D warnings passed.
• Real rust-analyzer smoke passed on the integration branch: cargo test -p mcpls-core --test integration_tests integration::rust_analyzer_tests::test_hover_on_u64_type -- --ignored --exact --nocapture.

[vcheckk]

Readiness note: keeping this PR as draft because the current branch is stacked on #180 and #181 / issues #104 and #108. It now passes local cargo test -p mcpls-core and cargo clippy -p mcpls-core --all-targets -- -D warnings, but it should be reviewed after the dependency chain or rebuilt/rebased once those land.