fix(sender): settle sends when the loop terminates

[Ehsan-hash]

Why

When the sender loop fails or is cancelled, concurrent requests can wait forever for a send that will never complete. Keep a terminal sender state so active, queued and later sends all finish.

What changed

• Propagate the original writer error to outstanding and later sends.
• Preserve cancellation during shutdown, including cancellation before the sender loop starts.
• Add sender lifecycle regressions and a two-request regression through Connection.

Scope

• Covers the private byte-stream sender and its callers; ordered graceful drain remains intact.
• No protocol, schema or transport API changes. Related to fix: drain pending futures on MessageSender loop termination #74, including the active and later sends that queue draining alone does not cover.

Blast Radius

Changes how pending sends terminate after writer failure or shutdown. Existing peer cancellation, stdio and HTTP/WebSocket tests pass.

Verification

• Unchanged source: 5 failed, 7 passed; fixed focused regressions: 12 passed.
• Affected RPC, cancellation, recovery and logging tests: 75 passed.
• make check passes; make test: 413 passed, 1 skipped on Python 3.12. The skipped Gemini integration was not enabled.

Developed and reviewed with AI assistance using Codex. No live model/provider calls were used for validation.