Stabilize session lifecycle E2E tests

[MackinnonBuck]

Summary

• run the Node handler-reentrancy detach check from session.idle, after the active turn has completed, so it tests disconnecting inside event dispatch without also racing active-turn cancellation
• detach the Python session before resuming it with custom-agent configuration, matching persisted-session ownership semantics and the stable .NET coverage
• leave replay snapshots unchanged

Root causes

The Windows Node failure was not an event-handler dispatch deadlock. The test detached on user.message while the turn was still active, so session.detach could wait on active-turn shutdown; when it remained pending, test teardown disposed the JSON-RPC connection and produced -32097.

The macOS Python failure resumed the same session while its original attachment was still live. Applying custom-agent configuration during that overlapping ownership transition could leave session.resume pending. The replay proxy's No stored conversations messages were interleaved output from another xdist worker, not a missing fixture for the timed-out request; timeout diagnostics showed the blocked RPC was session.resume.

Validation

• Node focused test: 20/20 consecutive passes on Windows
• Node session.e2e.test.ts: 37 passed, 2 skipped
• Node Prettier, ESLint, and TypeScript checks passed
• Python focused test: 20/20 consecutive passes for default transport and 20/20 for in-process
• Python create-then-resume sequence: 10/10 passes for default transport and 10/10 for in-process
• Python test_mcp_and_agents_e2e.py: 11/11 passes for default transport and 11/11 for in-process
• Python Ruff format/lint and ty check copilot passed

This is based directly on current main and is independent of #2544.

[Copilot]

Copilot review overview

🟢 Approval recommended

The targeted lifecycle sequencing changes are consistent with session semantics and introduce no unresolved issues.

Review tier: Balanced
Findings: None

What changed in this PR

Stabilizes session lifecycle E2E tests by avoiding overlapping session ownership and active-turn cancellation races.

Changes:

• Node disconnect test now runs from session.idle.
• Python resume test disconnects the original session first.

File	Description
nodejs/​test/​e2e/​session.e2e.test.ts	Tests handler-triggered disconnect after turn completion.
python/​e2e/​test_mcp_and_agents_e2e.py	Detaches before resuming with custom-agent configuration.

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