fix(weave): ignore unfinished calls in latency stats

[gtarpenning]

Summary

• Ignore latency values when ended_at precedes started_at, covering the epoch sentinel used by unfinished calls.
• Keep unfinished calls in request and error counts, and preserve valid zero-duration calls.
• Extend ClickHouse regression coverage for completed, unfinished, and zero-duration calls.

Testing

• 13 passed in the call-metrics query-builder suite.
• 12 passed across call-stats integration coverage, including the calls-complete regression case.
• Pre-push Ty, Pyright, mypy, import-linter, Ruff, and Fixit checks passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!