perf(bridge): remove translator mutex from tool requests

[vcheckk]

Closes #108.

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.
• I tested replaying the perf(bridge): reduce Mutex<Translator> hold time — release lock before client.request() #108 commit directly onto current origin/main; it conflicts in lib.rs, mcp/handlers.rs, mcp/server.rs, and transport.rs without the fix(bridge): split NotificationCache out of Translator lock to eliminate notification_pump starvation #104 lock split.
• Because I do not have push access to bug-ops/mcpls, I cannot set this PR's base to the fork branch used by fix(bridge): split notification cache lock #180 in a way that GitHub will show as an upstream stacked PR. Review after fix(bridge): split notification cache lock #180 lands, or rebase this branch after fix(bridge): split notification cache lock #180 is merged.

What changed

• Removes the outer translator mutex from MCP tool request handling so LSP requests do not serialize unrelated tools.
• Moves runtime mutation behind narrower translator-owned locks, primarily the document tracker.

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 / issue #104. It now passes local cargo test -p mcpls-core and cargo clippy -p mcpls-core --all-targets -- -D warnings, but it should either be reviewed after #180 or rebuilt/rebased once #180 lands.