fix(notebook): close environment leases on disposal

[ewen-poch]

Problem

Notebook runtime terminal disposal did not close its environment lease coordinator. A queued lease acquisition could therefore complete after the runtime crossed its disposal boundary, while releases from already-held leases could still mutate shutdown state.

Proposed change

• Dispose the environment lease coordinator synchronously when terminal runtime disposal begins.
• Add a regression test covering both a queued acquisition and a previously acquired lease.

Scope and non-goals

• Limited to Notebook environment-lease lifecycle behavior.
• No persisted data, renderer UI, or Specialist changes.

Acceptance criteria and validation

• Queued acquisition rejects after dispose(), held release returns false, and runtime disposal completes: covered by src/main/notebook/runtime-service.test.ts.
• npm run typecheck: passed.
• npm run lint: passed with 0 errors and the existing 23 warnings.
• npm test: 630 files passed, 15 skipped; 9,356 tests passed, 184 skipped.

Review focus

• Whether the lease coordinator closes at the terminal boundary before asynchronous teardown can yield.
• Whether the regression test captures both queued and held-lease behavior.

Related: #577

[github-actions]

Codex Review

Verdict: mergeable

No actionable findings.

Summary: No concrete merge-blocking defects found in the pull request changes. Branch and title checks are valid; review was static-only as requested.