feat(session): add compact action to session control tool

[1688mengdie]

Summary

Add the compact action to the session control tool family so a caller can compact a session context to reduce its token footprint and surface the applied compaction statistics (tokens/ratio/summary).

Plan

• Thread ContextCompactionOutcome through the manual compaction task so the finished outcome is returned instead of being discarded.
• Add compact_session_with_outcome returning BitFunResult<ContextCompactionOutcome>; compact_session_manually delegates to it for the Desktop compatibility API.
• Extend SessionControlAction with a Compact variant and its as_str mapping.
• Allow compacting the caller's own session (the sole self-mutation exception) while keeping owner/creator/ancestor authorization inline.
• Wire the Compact dispatch to the compact engine and return the applied stats.
• Extend the tool input schema/description and add compact validation tests.

AI-assisted output. Test extent: 已测 (tested).