event: account for raw rows in batch DML event size (#5960)

[ti-chi-bot]

This is an automated cherry-pick of #5960

What problem does this PR solve?

Issue Number: close #5959

Remote BatchDMLEvent instances keep their encoded row payload in RawRows
until the event collector calls AssembleRows. GetSize only accounted for
Rows, so dynamic stream memory control and byte-based batching treated the
remote row payload as zero bytes.

What is changed and how it works?

Make BatchDMLEvent.GetSize return the encoded RawRows length before rows
are assembled. After assembly, it continues to return the decoded chunk's
memory usage.

Extend the existing remote-row assembly unit test to verify size accounting
before serialization, after remote deserialization, and after row assembly.

Check List

Tests

• Unit test
• Manual test

Questions

Will it cause performance regression or break compatibility?

No compatibility impact. Remote batches may be split earlier when their actual
payload reaches the configured byte threshold, which is the intended behavior.

Do you need to update user documentation, design documentation or monitoring documentation?

No.

Release note

Fix dynamic stream memory and byte-batch accounting for remotely received DML batches.

Summary by CodeRabbit

• Bug Fixes
  • Improved batch event size reporting when row data is stored in its encoded form.
  • Preserved accurate in-memory size reporting after row data is decoded and assembled.
• Tests
  • Added coverage for size accounting before and after row data decoding.

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick:

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2. For pull requests to TiDB-x branches, it must have no failed tests.
3. AFTER it has lgtm and approved labels, please wait for the cherry-pick merging approval from triage owners.

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: wlwilliamx

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[wlwilliamx]

/test all