dm: keep S3 external ID in import-into source dir

[fgksgf]

Summary

• Keep Mydumper.SourceDir unchanged for DM import-into tasks, including S3 external-id.
• Remove the import-into-specific source-dir stripping added in dm: strip S3 external ID for import-into source dir #12670.
• Update the DM loader tests to assert import-into keeps the original source dir.

Why

DM import-into uses the same S3 location across dumpling, Lightning local metadata/file reads, and the final TiDB IMPORT INTO SQL. All of these paths need the keyspace role parameters to remain available.

Live dev validation on cluster 10492432391076049734 showed dumpling completed after the dataflow-service fix, but Lightning failed in the Load phase before submitting IMPORT INTO because Mydumper.SourceDir had already been stripped and the local import SDK could not assume the keyspace role.

TiDB IMPORT INTO now allows an explicit S3 external-id, so the correct fix is to keep the original S3 URI end to end instead of stripping it in Tiflow.

How

• Revert MakeGlobalConfig to always set lightningCfg.Mydumper.SourceDir = cfg.Dir.
• Remove the S3 external ID stripping helper from DM loader.
• Keep the backend selection behavior unchanged: LoadModeImportInto still maps to BackendImportInto.

Testing

• Ran gofmt on changed files.
• Not run: full go test; this PR was prepared from GitHub contents without a full Tiflow checkout in the current workspace.
• Runtime validation remains required after the DM/Tiflow image consumes this PR.

Risks and Reviewer Focus

• Please verify the current TiDB version used by this release branch accepts explicit S3 external-id in IMPORT INTO resource parameters.
• This change intentionally keeps the S3 URI unchanged for all consumers: dumpling, Lightning local source reads, and TiDB IMPORT INTO.

[ti-chi-bot]

Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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[gemini-code-assist]

Code Review

This pull request removes the logic that strips the S3 external ID from the source directory when using the BackendImportInto backend in the DM loader. The helper function stripS3ExternalIDFromSourceDir has been removed, and the corresponding unit tests have been updated to verify that the S3 external ID is preserved. No review comments were provided, and there is no additional feedback to offer.

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[ti-chi-bot]

@fgksgf: 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-dm-integration-test-next-gen	6afff10	link	true	/test pull-dm-integration-test-next-gen

Full PR test history. Your PR dashboard.

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