fix(pipeline): limit streaming aggregation memory without spill

[JackyWoo]

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:
spill_streaming_agg_mem_limit (default 256MB) was only applied when query-level
spill was enabled (enable_spill = true). When spill is disabled — which is still
a common deployment — the streaming aggregation operator now only limit the hastable by
row count but not data size, which may causing OOM on query with large agg_state queries.

Release note

None

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:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

Limit streaming aggregation memory without spill.

The value of spill_streaming_agg_mem_limit is a tradeoff between query time and memory.
The following is a query in my prodoction env.

Metric	Before	After	Change
Total runtime	67 s	76 s	+13.4%
Streaming agg memory / instance	992.99 MB	363.00 MB	-63.4%

• Does this need documentation?
  • No.
  • Yes.

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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