planner: fix unable to push down agg to tiflash when tidb_isolation_read_engines is set

[time-and-fate]

What problem does this PR solve?

Issue Number: close #31240

Problem Summary:

When tidb_isolation_read_engines doesn't contain tikv, the stream agg can't be pushed down to tiflash.

What is changed and how it works?

Make the logic in getStreamAggs be consistent with getHashAggs.

Add CopTiFlashLocalReadTaskType when the agg can't be pushed to tikv but can be pushed to tiflash.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• fzhedu
• xuyifangreeneyes

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/4775e0d4e2cd1a31ee3a0b301dd2dae16cc2381a

[time-and-fate]

/run-unit-test

[fzhedu]

/LGTM

[time-and-fate]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: b504b44

[ti-chi-bot]

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[time-and-fate]

/run-unit-test

[time-and-fate]

/run-unit-test

[time-and-fate]

/run-unit-test