fix(server): prevent unbounded bus entry growth for sandbox IDs

[johntmyers]

🏗️ build-from-issue-agent

Closes #27

Summary

Prevents unbounded memory growth caused by sandbox bus entries (TracingLogBus, PlatformEventBus, SandboxWatchBus) that were never cleaned up when sandboxes were deleted. Additionally, WatchSandbox now validates sandbox existence before subscribing to buses (preventing entries for non-existent IDs), and PushSandboxLogs validates sandbox existence once at stream open.

Changes Made

• crates/navigator-server/src/tracing_bus.rs: Added remove() method to TracingLogBus (cleans up per_id and tails maps) and remove() method to PlatformEventBus. Added 7 unit tests.
• crates/navigator-server/src/sandbox_watch.rs: Added remove() method to SandboxWatchBus. Added 3 unit tests.
• crates/navigator-server/src/sandbox/mod.rs: Wired bus cleanup into handle_deleted reconciler — calls remove() on all three buses after sandbox deletion. Updated spawn_sandbox_watcher to accept TracingLogBus parameter.
• crates/navigator-server/src/grpc.rs: Wired bus cleanup into delete_sandbox gRPC handler. Reordered watch_sandbox to validate sandbox existence before subscribing to buses (validate → subscribe → re-read snapshot). Added one-time sandbox validation at stream open in push_sandbox_logs.
• crates/navigator-server/src/lib.rs: Pass tracing_log_bus to spawn_sandbox_watcher.
• architecture/gateway.md: Documented bus cleanup and validation behavior.

Deviations from Plan

None — implemented as planned.

Tests Added

• Unit: 10 tests added across tracing_bus.rs (7 tests: remove cleans up maps, subscribe after remove creates fresh channel, remove closes active receivers, remove nonexistent is noop — for both TracingLogBus and PlatformEventBus) and sandbox_watch.rs (3 tests: remove cleans up, subscribe after remove creates fresh channel, remove nonexistent is noop)
• Integration: N/A
• E2E: N/A

Documentation Updated

• architecture/gateway.md: Added cleanup and validation notes to the bus section

Verification

• Pre-commit checks passing (license:check failure is pre-existing on main for tmp/network_checks.py)
• All 70 navigator-server tests passing (65 existing + 10 new, plus 5 integration)
• E2E tests (not applicable — no e2e/ changes)
• Architecture documentation updated