feat(transformations): add external data sources client API

[Khyat-Cognite]

CDF-28251

Summary

Slice 2 of 3 — splits PR #2713.

Adds the client API for managing Fabric OneLake external data sources under client.transformations.external_data_sources. Depends on PR #2714 (data classes).

What's in this PR

Production code

• cognite/client/_api/transformations/external_data.py — async list, upsert, delete, verify_usability
• cognite/client/_sync_api/transformations/external_data.py — generated sync wrapper
• cognite/client/_api/transformations/__init__.py + sync __init__.py — wire external_data_sources sub-API
• cognite/client/_cognite_client.py — Sphinx doc-build stub
• cognite/client/testing.py — CogniteClientMock / AsyncCogniteClientMock autospec wiring

Tests (co-located)

• tests/tests_unit/test_api/test_transformation_external_data.py — HTTP mock tests for all four endpoints
• tests/tests_integration/test_api/test_transformations/test_external_data.py — Fabric-gated lifecycle (upsert → list → verify_usability → delete); skips without FABRIC_* env vars

SDK surface

client.transformations.external_data_sources.list()
client.transformations.external_data_sources.upsert(source)
client.transformations.external_data_sources.delete("my-source")
client.transformations.external_data_sources.verify_usability("my-source")

Upsert always sends "format": "one_lake".

What came before

PR	Status	Scope
1 — Data model	#2714	Data classes, IAM capability, serialization unit tests

What comes next

PR	Branch	Scope
3 — Docs	feat/onelake-edc-docs	User guide, quickstart, API reference page

Merge #2714 first, then this PR (rebase onto master after #2714 lands, or merge stacked).

Test plan

• pytest tests/tests_unit/test_api/test_transformation_external_data.py
• verify-sync-codegen CI green
• Integration tests skip without Fabric credentials

Parent

Replaces the API portion of #2713. Close #2713 after all three slices are merged to master.

[codecov]

Codecov Report

❌ Patch coverage is 82.81250% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.70%. Comparing base (f7888df) to head (d0ccc58).
⚠️ Report is 1 commits behind head on feat/onelake-edc-data-model.

Files with missing lines	Patch %	Lines
...est_api/test_transformations/test_external_data.py	37.73%	33 Missing ⚠️

Additional details and impacted files

@@                      Coverage Diff                      @@
##           feat/onelake-edc-data-model    #2715    +/-   ##
=============================================================
  Coverage                        93.69%   93.70%            
=============================================================
  Files                              506      510     +4     
  Lines                            51373    51573   +200     
=============================================================
+ Hits                             48133    48325   +192     
- Misses                            3240     3248     +8     

Files with missing lines	Coverage Δ
cognite/client/_api/transformations/__init__.py	75.70% <100.00%> (+0.46%)	⬆️
...gnite/client/_api/transformations/external_data.py	100.00% <100.00%> (ø)
cognite/client/_cognite_client.py	95.83% <100.00%> (+0.01%)	⬆️
...gnite/client/_sync_api/transformations/__init__.py	94.64% <100.00%> (+0.19%)	⬆️
.../client/_sync_api/transformations/external_data.py	100.00% <100.00%> (ø)
cognite/client/testing.py	100.00% <100.00%> (ø)
...unit/test_api/test_transformation_external_data.py	100.00% <100.00%> (ø)
...est_api/test_transformations/test_external_data.py	37.73% <37.73%> (ø)

... and 9 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[Khyat-Cognite]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces the TransformationExternalDataAPI (and its synchronous counterpart) to manage Fabric OneLake external data sources for transformations, supporting listing, upserting, deleting, and verifying usability. The review feedback suggests improving type safety by adding @overload signatures to the upsert method and ensuring integration tests are not silently skipped in CI environments by checking for existing environment variables before looking for a .env file.

[gemini-code-assist]

Code Review

This pull request introduces the TransformationExternalDataAPI (and its sync counterpart) to manage Fabric OneLake external data sources for transformations, including list, upsert, delete, and usability verification endpoints. The feedback suggests adding support for the ignore_unknown_ids parameter in the delete method to maintain consistency with the rest of the SDK, updating the unit tests accordingly, and refactoring the integration test teardown to avoid masking potential setup failures in the finally block.