integration test: handle emulator to enable using spanner omni emulator for local and testing

[yiyche]

I'm working on adding an emulator based test for DCP.

prior to this PR, it was impossible to run the loader pipeline against a local Spanner emulator (like Spanner Omni) because the loader had no way to receive the emulator's port/endpoint or bypass Google Cloud authentication checks.

[codacy-production]

Not up to standards ⛔

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[gemini-code-assist]

Code Review

This pull request adds support for connecting to a local Spanner emulator in both the Java ingestion pipeline and the Python ingestion-helper workflow, including safety checks to prevent emulator usage in production environments. A review comment correctly points out that the Python Spanner client fails to override the API endpoint with the emulator host when configured, and that its production environment check is case-sensitive, which could bypass the safety guardrail.