[Python] Keep BigQuery load jobs distinct for table aliases

[bvolpato]

Python BigQuery FILE_LOADS can silently omit files when dynamic destinations use different spellings of the same table, such as project:dataset.table and dataset.table. File grouping keeps those strings separate, but load-job IDs hash the resolved table and collide. BigQuery then treats a later submission as an existing job.

This change uses the destination identity from file grouping to generate load-job IDs. Each group gets its own load job and temporary table, while retries remain deterministic. Callback arguments and copy write-disposition handling are preserved. The tox change applies Beam's existing GRPC_ENABLE_FORK_SUPPORT=0 workaround before Python imports gRPC.

The FILE_LOADS unit suite passed after merging master, using fake jobs and stubbed credential discovery. No live BigQuery service was used. Run the Python cloud unit suite with:

./gradlew :sdks:python:test-suites:tox:py312:testPy312Cloud

Job IDs change for noncanonical destination spellings. Replaying unfinished work across an in-place SDK update can resubmit completed work and duplicate rows. Canonical project:dataset.table destinations keep their existing IDs, and retries within either version remain stable.

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• Replace these instructions with a description of your change and why you have made it.
• Mention the appropriate issue in your description (for example: addresses #123 or fixes #456).
• Update CHANGES.md with noteworthy changes.

See CONTRIBUTING.md for contribution guidelines.

[bvolpato]

I checked the macOS Python 3.14 failure. JavaJarServerTest.test_classpath_jar fails because the Java subprocess exits with SIGABRT; that test and its implementation are unchanged by this PR. The FILE_LOADS tests are skipped in that CI lane because it has no GCP dependencies. All 38 FILE_LOADS unit tests pass locally with those dependencies installed.

Could someone with rerun access retry the macOS job? My fork account cannot rerun upstream Actions. Thanks!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.75%. Comparing base (76f6e6d) to head (48a3f65).
⚠️ Report is 143 commits behind head on master.

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #40001       +/-   ##
=============================================
+ Coverage     56.16%   79.75%   +23.59%     
=============================================
  Files          1124      487      -637     
  Lines        178336    82280    -96056     
  Branches       1489        0     -1489     
=============================================
- Hits         100170    65626    -34544     
+ Misses        75645    16654    -58991     
+ Partials       2521        0     -2521     

Flag	Coverage Δ
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java	?
python	79.75% <ø> (+0.20%)	⬆️

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