consumer: skip data file by global checkpointTs in storage consumer (#4886)

[ti-chi-bot]

This is an automated cherry-pick of #4886

Added functionality to retrieve and update global checkpoint timestamp from metadata file.

What problem does this PR solve?

Issue Number: close #4885

What is changed and how it works?

file discovery is now fenced by the storage metadata checkpoint, instead of discovering every visible schema.json and .index file immediately.

• Before this patch, getNewFiles() scanned all visible schema/index files
  and immediately inserted them into the consumer’s in-memory seen state
  (tableDMLIdxMap / tableDefMap), then diffDMLMaps() decided what was “new”.
• That meant a later-version DDL file could be discovered first, executed in
  cmd/storage-consumer/consumer.go:596, and advance tableDDLWatermark. If an
  older-version DML index was only discovered in the next round, it hit the
  unchanged stale-DML branch at cmd/storage-consumer/consumer.go:678 and got
  dropped.
• After this patch, discovery itself is checkpoint-aware. Files whose
  TableVersion is ahead of the current global checkpoint are skipped before
  they enter the seen maps.
• That detail matters: skipped files are not marked as seen. So when the
  checkpoint later advances, those files are discovered for the first time
  and diffDMLMaps() still reports them as genuinely new.
• The stale DDL/DML logic in cmd/storage-consumer/consumer.go:641 and cmd/
  storage-consumer/consumer.go:680 is unchanged. The fix works by preventing
  future schema versions from being discovered too early, so the watermark
  no longer runs ahead of older-version DML discovery.

Check List

Tests

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

Questions

Will it cause performance regression or break compatibility?

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

Release note

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[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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[ti-chi-bot]

@wk989898 This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

@ti-chi-bot: ## If you want to know how to resolve it, please read the guide in TiDB Dev Guide.

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[ti-chi-bot]

Cherry-pick conflicts appear resolved; removing the do-not-merge/hold label.

[ti-chi-bot]

@ti-chi-bot: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-error-log-review	266b510	link	true	/test pull-error-log-review

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: wk989898

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