[transforms] Per-session dynamic tool visibility #65

Depends on: Issue #61

Summary

Let a tool start hidden and be enabled or disabled per session from a handler, for example to unlock admin tools after login.

Reference

• ts:core/tool.ts: visibility?: 'visible' | 'hidden', defaultVisible.
• ts:core/types.ts: ctx.enableTools(names), ctx.disableTools(names).
• ts:core/server.ts:
  • sessionIsolationKey(transportSessionId, verifiedSubject).
  • With no transport session id, enableTools logs a warning and does nothing.
  • Unknown tool names are warned about and skipped.
• ts:core/transforms/visibility/ (visibility.transform.ts, session-store.ts, with TTL and LRU eviction).

Scope

• visibility / default_visible on @tool.
• ExecutionContext.enable_tools(names) and disable_tools(names).
• VisibilityTransform plus a SessionVisibilityStore with TTL and max-entries eviction.
• Emit notifications/tools/list_changed when visibility changes, where the transport supports it.
• Decision required: Python's default protocol era (auto) is sessionless, so the TypeScript rule of "no-op without a session id" would no-op by default. Pick one before implementing, and record it in the PR:
  • (a) match TypeScript and document that visibility needs a sessionful era;
  • (b) fall back to the verified token subject;
  • (c) another key.

Acceptance criteria

• Hidden tools are absent from tools/list and tools/call rejects them as unknown, until they are enabled for that session.
• Two concurrent sessions are isolated.
• A session id supplied by a client cannot read another session's state; the key includes the verified subject.
• Expired sessions are evicted (fake clock).