fix(runtime): terminate fill-missing time spines for unaligned bounds

[eminemead]

• SelectTimeRangeBins in the starrocks, clickhouse, pinot, druid, and snowflake dialects looped forever when the truncated start never equalled an unaligned end (t != end), allocating unbounded SQL until the process was OOM-killed.
• A shared timeutil.TimeRangeBins now truncates the start, iterates half-open with t.Before(end), and errors above 1500 actual bins. Those dialects only format that slice.
• DuckDB / BigQuery / Databricks native-SQL paths are unchanged; the ApproximateBins pre-check in ast.go stays as a fast-fail.
• Live-verified on an unaligned UTC day fillMissing request: HTTP 200 in under a second with flat memory (previously hung until timeout while resident memory grew to tens of GB).

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

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Developed in collaboration with Claude Code