Fix streaming skip accounting and next_extraction task leak

[KRRT7]

Summary

Follow-up to #265. Fixes two issues identified during review:

• Skip accounting gap. ingest_email.py only reported generator-level skips (counters["skipped"]). Batch-level skips from _filter_ingested (result.messages_skipped) were never surfaced in the final summary. The two populations are disjoint — a source caught by the generator never reaches the batch layer. The summary now reports total_skipped = counters["skipped"] + counters["batch_skipped"]. Survives ^C since on_batch_committed fires per committed batch.

• next_extraction task leak. In _submit_batch, if _drain_commit() raises after next_extraction = asyncio.create_task(...) but before it's awaited, the task leaked. Promoted next_extraction to a nonlocal (pending_extraction) so the except BaseException block can cancel it alongside pending_commit.

Changes

• tools/ingest_email.py: Add batch_skipped counter, track result.messages_skipped in on_batch_committed, report combined total in summary
• src/typeagent/knowpro/conversation_base.py: Track pending_extraction, cancel in except block
• tests/test_add_messages_streaming.py: 4 new tests covering both cancellation paths and edge cases

Test plan

• make format check test passes (701 tests)
• Coverage for conversation_base.py: 94% → 96%
• New test: pending_extraction cancelled when prior commit raises during _drain_commit
• New test: pending_commit cancelled when message iterator raises
• New test: empty iterator returns zeros
• New test: messages with empty text_chunks skip extraction entirely