checker(dm): improve same table precheck error

[GMHDBJD]

What problem does this PR solve?

Issue Number: close #12737

DM precheck can report a confusing raw table-router error when route rules only differ by table-name case under case-insensitive matching, for example:

RawCause: table test.T matches more than one rule

This hides the existing clearer same-target-table diagnostic.

What is changed and how it works?

Add a checker-side preflight for case-insensitive route rules before building the regular-expression table router.

The preflight detects source table patterns that collide after case folding and map to target tables that differ only by case, then returns the existing ErrTaskCheckSameTableName error with actionable details such as:

same target table `test`.`T` vs `test`.`t`

This preserves the clearer DM precheck error path and avoids exposing the router's ambiguous duplicate-rule message.

Check List

Tests

• Unit test

Commands run:

go test ./dm/checker -run 'TestSameTargetTableDetection(BeforeRouteDupMatch)?$' -count=1
go test ./dm/checker -count=1
make fmt
git diff --check

Questions

Will it cause performance regression or break compatibility?

No. The change only adds a lightweight route-rule validation step during checker initialization and reuses the existing same-table-name error type.

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

No.

Release note

Fix a confusing DM precheck error message when case-insensitive route rules for the same source table differ only by table-name case.

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[gemini-code-assist]

Code Review

This pull request introduces a pre-check (sameTargetTableNameDetectionForRouteRules) to detect case-insensitive target table name conflicts in route rules before the table router is initialized. This prevents confusing "matches more than one rule" errors from being thrown by regexprrouter during precheck. Feedback on the changes suggests changing the condition else if nameO != name to a simple else to ensure that duplicate or colliding rules mapping to the exact same target table name (including case) are also caught and reported.

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[gemini-code-assist]

If two route rules collide under case-insensitive matching but map to the exact same target table name (including case), nameO == name will be true. Because of the else if nameO != name condition, this collision will not be detected by this preflight check, and the user will still encounter the confusing matches more than one rule error from regexprrouter.

Changing this to a simple else ensures that any duplicate or colliding rules for the same source table pattern are caught and reported with a clear error message.

Suggested change

	if nameO, ok := tableNameSets[nameL]; !ok {
	tableNameSets[nameL] = name
	} else if nameO != name {
	messages = append(messages, fmt.Sprintf("same target table %v vs %s", nameO, name))
	}
	if nameO, ok := tableNameSets[nameL]; !ok {
	tableNameSets[nameL] = name
	} else {
	messages = append(messages, fmt.Sprintf("same target table %v vs %s", nameO, name))
	}

[GMHDBJD]

Thanks, this is reasonable. Updated the preflight to catch any case-insensitive source-rule collision, including collisions that map to the exact same target table name, and added regression coverage for that case.