Defer code generator validation until transform submission

[gordonwatts]

Goal: make sure no internet is needed for a cached query

Summary

• lazily fetch and cache ServiceX code generators
• validate code generators only when submitting uncached transforms
• provide list of supported code generators when an unsupported one is requested
• add tests covering cached vs uncached transform paths

Testing

• black tests/test_databinder.py servicex/query_core.py
• flake8 tests/test_databinder.py servicex/query_core.py
• pytest

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https://chatgpt.com/codex/tasks/task_e_689df04bede4832088ce0db7ccf3044b

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.04%. Comparing base (1413cd7) to head (3b2a863).
⚠️ Report is 23 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #644      +/-   ##
==========================================
+ Coverage   97.19%   98.04%   +0.84%     
==========================================
  Files          29       29              
  Lines        1959     2043      +84     
==========================================
+ Hits         1904     2003      +99     
+ Misses         55       40      -15     

Flag	Coverage Δ
unittests	98.04% <100.00%> (+0.84%)	⬆️

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[gordonwatts]

Marked as draft until I come back from vacation. If urgent and someone else wants to tackle it, then go for it!

[Copilot]

Pull Request Overview

This PR defers ServiceX code generator validation until transform submission to optimize performance and avoid unnecessary network calls for cached queries.

• Lazy loading of code generators with caching to avoid network calls when ServiceX client is instantiated
• Code generator validation moved from client instantiation to transform submission time
• Enhanced error messages that list available code generators when an unsupported one is requested

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
servicex/servicex_client.py	Implements lazy loading of code generators with caching and removes upfront validation
servicex/query_core.py	Adds async code generator validation with helpful error messages during transform submission
tests/test_servicex_dataset.py	Updates test mocks to include code generator configuration
tests/test_dataset.py	Updates test mocks to include code generator configuration
tests/test_databinder.py	Updates test structure and adds validation for error messages with code generator lists
tests/test_code_generator_fetch.py	Adds new tests covering cached vs uncached transform paths

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[BenGalewsky]

I don't think the code in query_core is correct?

[BenGalewsky]

Cheers