sync-diff-inspector: filter auto snapshot syncpoint by changefeed

[pingyu]

What problem does this PR solve?

Issue Number: close #898

sync-diff-inspector currently resolves snapshot = "auto" by reading the latest row from tidb_cdc.syncpoint_v1 globally. When multiple TiCDC changefeeds write syncpoint rows into the same downstream table, this can select another changefeed's syncpoint and produce false-positive data mismatches.

What is changed and how it works?

This PR adds an optional syncpoint-changefeed task config for sync-diff-inspector. When both source and target snapshots are set to "auto" and this option is configured, sync-diff-inspector filters tidb_cdc.syncpoint_v1 by that changefeed before selecting the latest syncpoint row. If the option is omitted, the existing global latest-row behavior is preserved.

The change also updates the config template, includes the new field in the task config hash, and adds unit coverage for config parsing plus filtered syncpoint lookup behavior.

Check List

Tests

• Unit test (go test ./sync_diff_inspector/config ./sync_diff_inspector/source)

Code changes

• Has exported variable/fields change

Release Note

sync-diff-inspector supports filtering auto snapshot syncpoint lookup by TiCDC changefeed with the `syncpoint-changefeed` configuration.

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