Default source: GIT on git_source job tasks

[pietern]

Changes

Restore ApplyDefaultTaskSource as a shared resource mutator that sets source: GIT on the tasks of a job with a git_source (for the task types that support the field: dbt_task, gen_ai_compute_task, notebook_task, spark_python_task, sql_task.file, including for_each_task, and only when the user did not set it). It runs after PythonMutator, so it reflects any git_source/tasks that Python code added or removed, is not exposed to Python, and is still observable in bundle validate/summary. Non-git jobs are left untouched, and an explicit git_source: null is treated as absent (matching TranslatePaths). tfdyn is unchanged and becomes an idempotent no-op.

Why

Deploying a job with a git_source and a spark_python_task (repo-relative python_file) failed on the direct engine with Invalid python file reference: <path> (400 INVALID_PARAMETER_VALUE), while Terraform deployed it fine. Path translation is intentionally skipped for git_source jobs, so task file paths stay relative to the Git repo. The backend treats a task as git-sourced only when its source is explicitly GIT; it does not infer that from the job-level git_source. Terraform stamped source: GIT in tfdyn, but the direct engine sent no source, so the backend read the relative path as a workspace path and rejected it. This was latent since the defaulting moved from a shared mutator into tfdyn (#3528) and only became user-visible once the direct engine became the default. See #3359 and #3528 for the history.

Tests

Adds a direct-only acceptance test that reproduces the failure, plus testserver validation that mirrors the backend's python_file check so the test fails without the fix. Adds unit tests covering git/non-git/null/for_each and explicit-source cases. The only existing golden that changes is task-source/out.git_job.direct.txt, which now includes source: GIT. Verified end-to-end against e2-dogfood (direct deploy of a git-sourced spark_python_task now succeeds).

This pull request and its description were written by Isaac.

[denik]

Do we need another test for PyDABs? I vaguely remember there was some issue between this mutator and PyDABs resources. cc @andrewnester

[eng-dev-ecosystem-bot]

Integration test report

Commit: 94fbafa

Run: 35339379831

	Env	✅​pass	🙈​skip	Time
✅​	aws linux	276	17	5:30
✅​	aws windows	278	15	5:32
✅​	azure linux	275	17	5:01
✅​	azure windows	277	15	4:31
✅​	gcp linux	276	17	5:15
✅​	gcp windows	278	15	5:53

Top 3 slowest tests (at least 2 minutes):

duration	env	testname
5:29	aws windows	TestAccept
3:55	gcp windows	TestAccept
3:41	azure windows	TestAccept