fix(runtime): warn near guest execution slot capacity

[SparshSunilNaik]

Summary

• Track process-wide guest execution admission slots through the existing limit registry.
• Emit the standard near-capacity warning before the hard execution limit is reached.
• Expose current depth, high-water mark, and capacity through the existing resource snapshot infrastructure.
• Preserve the existing fail-fast admission and release behavior.

Testing

• cargo test -p agentos-bridge queue_tracker
• cargo test -p agentos-v8-runtime session::tests::guest_execution_slots_are_tracked_and_warn_near_capacity --lib
• cargo check -p agentos-bridge -p agentos-v8-runtime
• cargo fmt --check
• git diff --check

session::tests::begin_destroy_session_removes_entry_before_finish also fails identically on clean base 1bf32804dbd36678dd536bfe68dbd773dad76d1b, so that failure is unrelated to this change.

Closes #1912