[feature](cache) Add session variable to keep INSERT source scans in the normal file cache queue

[raghav-reglobe]

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

NereidsCoordinator.setForInsert() unconditionally sets disableFileCache(true), so the source scan of every INSERT ... SELECT / CTAS is classified into the disposable file cache queue (the comment explains the intent: keep one-shot load scans from polluting the normal LRU).

That one-shot assumption doesn't hold for a common warehouse shape: recurring ETL. Hourly dbt-style builds re-read the same source tables (in our case Iceberg external tables) every cycle — but because the disposable queue is capped at 5% of the cache and hits never promote, those scans re-fetch from remote storage every hour, forever, no matter how large the file cache is.

This PR adds an opt-in session variable, enable_file_cache_for_insert_source (default false — existing behavior is unchanged byte-for-byte). When enabled, the insert source scan uses the normal cache queue. The broker-load constructor path is deliberately untouched (no ConnectContext, genuinely one-shot).

Design notes: we considered conditioning on the target catalog instead of a variable, but the pollution question is a property of the source scan's reuse pattern, not the target — a variable lets the operator state that intent directly. Automatic frequency-based promotion between queues would be the ideal long-term answer; that's out of scope here.

Measured in production (recurring hourly ETL over Iceberg external tables, 280GB file cache per BE): after enabling, the ETL account's remote-read rate dropped ~8x within hours (23.5 GB/h -> 3 GB/h), byte-level cache hit rate rose from 64% to 78% and climbing, and fleet block hit-ratio gained ~5pp.

Release note

New session variable enable_file_cache_for_insert_source (default false): when enabled, source scans of INSERT ... SELECT / CTAS use the normal file cache queue instead of the disposable queue.

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    Behavior is opt-in and default-off (existing behavior byte-identical). Manually verified in a production deployment: with the variable enabled, insert source scans populate the normal queue (normal_queue_cache_size growth, disposable evict flatline in BE bvars) and remote reads drop as described above.

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.
    Will add the session variable to the docs site once the approach is confirmed by reviewers.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

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