Ensure MCP launch resources are cleaned up

[tninja]

Summary

This PR adds lifecycle management for temporary resources created by ai-code-mcp-agent-prepare-launch.

Claude Code launches create a temporary MCP configuration file, but the previous cleanup function only unregistered the MCP session. The temporary file could therefore remain on disk after the session ended.

Changes

• Add TDD coverage for Claude MCP temp config cleanup.
• Add a test proving cleanup is idempotent.
• Return command metadata with tracked runtime files from backend command injection.
• Centralize cleanup in an idempotent helper that removes runtime files and unregisters the MCP session exactly once.

Impact

Claude Code MCP temporary configuration files are now removed when the session cleanup function runs. Other MCP-enabled backends keep their existing command behavior.

Validation

• Added focused ERT tests in test/test_ai-code-mcp-agent.el.
• The execution environment used to prepare this PR does not include Emacs, so the tests were not run locally; GitHub CI should provide the executable verification.

Follow-up

Startup-failure cleanup in the shared terminal/session infrastructure is intentionally left for a separate focused change, so this PR remains limited to resources owned by ai-code-mcp-agent.

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Clean the prepared launch when session startup fails

The cleanup added here only covers reuse of a live session. For a fresh Claude Code launch whose process is nil or exits during the initialization delay—for example, when the CLI cannot start—ai-code-backends-infra--create-new-session calls ai-code-backends-infra--handle-session-start-failure without invoking cleanup-fn, and no sentinel was installed to invoke it later. The newly created MCP configuration file is therefore still leaked on this failure path; ensure launch cleanup also runs when session creation fails.

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