fix: dispose HTTP batch server sessions

[irvinebroque]

An HTTP batch is a one-time conversation. When its response is ready, the server should close the conversation and release the objects it used. Today, successful batches can leave that cleanup undone.

The TODO predates the mechanism this patch uses:

• August 8, 2025: HTTP batching was added, including the cleanup TODO. Disposing the main stub did not yet close the session.
• August 12, 2025: main-stub disposal gained session shutdown. The HTTP handlers were not connected to that mechanism.

This patch makes that connection. Both handlers dispose the main stub in finally, after waiting for requested results and capturing the response. Cleanup also runs on failure. It reuses session shutdown rather than introducing another disposal mechanism.

[github-actions]

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