Document + modifier syntax in bundle run --only help

[afberg]

Changes

bundle run --only takes a comma-separated list of task keys to run a subset of a job's tasks. Its help text did not mention that a task key can be prefixed or suffixed with + to also pull in upstream/downstream tasks (the value is forwarded to the run-now only API field, which interprets the modifiers). This documents that syntax in the flag's help:

• +my_task: run my_task and everything upstream of it
• my_task+: run my_task and everything downstream of it
• +my_task+: both

Adresses #4244

Tests

• New acceptance/bundle/run/jobs/partial_run case asserts --only +task_1,task_2+ is forwarded unchanged to the run-now request body ("only": ["+task_1", "task_2+"]), proving the documented + syntax is not mangled or rejected client-side.
• Regenerated the bundle run --help golden for the updated --only description.

This pull request and its description were written by Isaac.

[github-actions]

Approval status: pending

/acceptance/bundle/ - needs approval

Files: acceptance/bundle/help/bundle-run/output.txt, acceptance/bundle/run/jobs/partial_run/output.txt, acceptance/bundle/run/jobs/partial_run/script
Suggested: @denik
Also eligible: @janniklasrose, @anton-107, @pietern, @andrewnester, @shreyas-goenka, @lennartkats-db

/bundle/ - needs approval

Files: bundle/run/job_options.go
Suggested: @denik
Also eligible: @janniklasrose, @anton-107, @pietern, @andrewnester, @shreyas-goenka, @lennartkats-db

General files (require maintainer)

Files: NEXT_CHANGELOG.md
Based on git history:

• @denik -- recent work in ./, acceptance/bundle/run/jobs/partial_run/

_{Any maintainer (@andrewnester, @anton-107, @denik, @pietern, @shreyas-goenka, @simonfaltum, @renaudhartert-db) can approve all areas.
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• PR number: 5760
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