fix: flush FilterExec output for unbounded inputs - #24353

Which issue does this PR close?

• Part of Document streaming usecase (like UNBOUNDED tables) #9016.

Rationale for this change

While prototyping the minimal push-based streaming example proposed in #9016, I found that FilterExec could buffer small filtered results indefinitely when its input was unbounded. Because an unbounded source may never finish, consumers could not observe those results while continuing to push input batches.

This PR extracts the smallest prerequisite fix so the streaming example and user-guide documentation can follow as separate, focused PRs.

This is complementary to #23856: that PR changes how filtered batches are supplied to the coalescer, while this PR controls when buffered rows become observable for an unbounded input.

What changes are included in this PR?

• Add a non-final LimitedBatchCoalescer flush operation that emits buffered rows while still allowing subsequent input.
• Preserve idempotent coalescer finalization.
• Flush FilterExec output after each input batch when its input is unbounded.
• Preserve the existing coalescing behavior for bounded inputs.
• Preserve existing fetch-limit behavior.
• Add regression coverage for:
  • emitting results while an unbounded input remains open;
  • multiple input pushes;
  • a fully filtered batch followed by a matching batch;
  • bounded-input coalescing;
  • flushing without preventing subsequent input.

Planned follow-up PRs:

1. Add the simplest possible push-based streaming example under datafusion-examples, showing how to feed batches into a running query and consume results incrementally.
2. Add a “Using DataFusion for streaming” section to the library user guide, based on that example and documenting relevant boundedness and execution considerations.

Are these changes tested?

Yes.

• Verified that the new unbounded-input regression test fails before the implementation and passes afterward.
• cargo test -p datafusion-physical-plan
• cargo test -p datafusion
• cargo test -p datafusion-cli
• cargo test --profile=ci --test sqllogictests
• Contributor-guide extended workspace test suite with avro,json,backtrace,extended_tests,recursive_protection,parquet_encryption
• cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• Rust documentation, license-header, and typo checks

Are there any user-facing changes?

Yes. FilterExec now emits filtered results incrementally for unbounded inputs instead of waiting for the target batch size or input completion.

Bounded-input coalescing remains unchanged. There are no public API or breaking changes.