feat(frontend): disallow backfill order with cross-db snapshot backfill

[kwannoel]

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What's changed and what's your intention?

Added a check to prevent using backfill order control with cross-database snapshot backfill operations. This change:

1. Adds a new has_cross_db_snapshot_backfill flag to the BuildFragmentGraphState struct to track when cross-database snapshot backfill is used
2. Sets this flag when a cross-database snapshot backfill stream scan is encountered
3. Adds validation logic to return a clear error message when users attempt to combine backfill order control with cross-database snapshot backfill

This prevents users from attempting unsupported combinations of features and provides a clear error message explaining the limitation.

I've updated the release note of fixed backfill order control to mention this: #20967.

Checklist

• I have written necessary rustdoc comments.

[kwannoel]

• feat(frontend): disallow backfill order with cross-db snapshot backfill #22300 👈 (View in Graphite)
• feat(stream): support snapshot backfill in backfill order control #22215
• feat(stream): support source backfill in backfill order control #22199
• feat(catalog): support source backfill progress in rw_fragment_backfill_progress #22218
• feat(frontend): add source backfill progress tracking #22217
• feat(frontend): support fragment level progress for snapshot backfill #22166
• feat(catalog): add fragment backfill progress view #22124
• feat(catalog): add rw_table_scan system catalog #22123
• feat(frontend): support querying internal backfill table progress #22122
• main

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[wenym1]

LGTM. May create an issue to track the follow-up.

[kwannoel]

LGTM. May create an issue to track the follow-up.

Tracked in #20861