allow for configuring pr preview default start and min intervals separately

[gjesse]

Summary

This updates SQLMesh PR preview planning so forward-only preview intervals can be controlled independently from the regular PR backfill window.

The GitHub bot now supports separate preview defaults for forward-only changes, allowing PR environments to keep broad default backfills where needed without forcing large preview builds for massive forward-only models.

This also fixes a related planning issue where preview fallback dates could accidentally constrain explicitly selected non-preview backfills when preview models were present. Explicit backfill_models selections now keep their normal model-derived interval behavior, while preview fallback starts are only applied when SQLMesh is implicitly selecting the backfill scope.

Changes

• Add GitHub bot config for forward-only PR preview windows:
  • default_pr_preview_start
  • pr_preview_min_intervals
• Pass preview-specific options through PR planning.
• Ensure forward-only previews can be capped independently from default_pr_start.
• Do not bind explicit non-preview backfills to short preview intervals when preview models are present.
• Cover both forward-only and non-forward-only preview paths with regression tests.

Validation

• Added focused unit coverage for the new planner behavior
• Added GitHub bot config/controller test coverage
• Ran targeted pytest and ruff checks

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the DCO

[gjesse]

I am adding to this to also fix #5789, as they are very closely related

[StuffbyYuki]

@gjesse Thank for the PR! A couple of things:

• For skip_pr_backfill=False shoud affect implicit default_pr_start calculation #5789: when skip_pr_backfill=False, default_pr_start=None, and there's a forward-only preview, this still sets plan start to preview_start ("yesterday") for implicit selection (backfill_models is None).

  That caps regular backfills to 1 day even though previews are already bounded separately via _forward_only_preview_start().

  Before this PR, implicit start was default_start or yesterday, so model-derived start from prod was used when available. Now it's always preview_start ("yesterday" by default from the bot).

  Since previews no longer need plan start, should we stop promoting preview_start to _start here? For example:

  • don't set _start from preview_start for forward-only previews at all, or
  • only do it when skip_backfill=True, and when skip_backfill=False use default_start / model-derived intervals for backfills

• Could we add a test for the skip_pr_backfill=False shoud affect implicit default_pr_start calculation #5789 scenario?

  • backfill_models=None (implicit selection)
  • skip_backfill=False
  • no explicit start / default_pr_start=None
  • forward-only preview model present

[gjesse]

@gjesse Thank for the PR!

glad to help! great catch on that one ;)

[StuffbyYuki]

@gjesse Can you look into cicd-tests-windows and style-and-cicd-tests? You can ignore test-dbt-versions. Also DCO checks are missing