fix(bundle/direct): construct UpdateApp update_mask dynamically based on changed fields (#6400)

[Shushankranjan]

Changes

• Dynamically construct update_mask in ResourceApp.DoUpdate using collectUpdatePathsWithPrefix(entry.Changes, "").
• Filter out non-API / deploy-only fields (source_code_path, config, git_source, lifecycle, lifecycle.*) from update_mask.
• Removed the static, hardcoded updateMask variable string in bundle/direct/dresources/app.go.
• Updated the acceptance test request recording acceptance/bundle/resources/apps/update/out.requests.direct.json.
• Added a changelog fragment in .nextchanges/bundles/app_update_mask.md.

Why

Previously, ResourceApp.DoUpdate sent a fixed update_mask string containing all updatable apps.App fields (including compute_size, compute_min_instances, compute_max_instances, etc.) on every UpdateApp call regardless of what was specified in the bundle config. Because apps.App fields use omitempty, omitted fields were dropped from the request body while still being specified in update_mask. Building the update_mask dynamically ensures it strictly matches the fields modified in entry.Changes.

Tests

• Updated acceptance/bundle/resources/apps/update/out.requests.direct.json fixture to verify that updating only description yields "update_mask": "description".
• Verified TestAppDoUpdate_UpdateMaskHasAllFields in bundle/direct/dresources/app_test.go.

[github-actions]

Approval status: pending

/acceptance/bundle/ - needs approval

Files: acceptance/bundle/resources/apps/update/out.requests.direct.json
Suggested: @denik
Also eligible: @andrewnester, @shreyas-goenka, @janniklasrose, @pietern, @anton-107, @lennartkats-db

/bundle/ - needs approval

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

General files (require maintainer)

Files: .nextchanges/bundles/app_update_mask.md
Based on git history:

• @denik -- recent work in .nextchanges/bundles/, bundle/direct/dresources/, acceptance/bundle/resources/apps/update/

_{Any maintainer (@andrewnester, @anton-107, @denik, @pietern, @shreyas-goenka, @simonfaltum, @renaudhartert-db, @janniklasrose, @lennartkats-db, @rugpanov, @rclarey) can approve all areas.
See OWNERS for ownership rules.}

[github-actions]

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Inputs:

• PR number: 6424
• Commit SHA: 59c46b44d40072ddb3ec0db425f25aba7336f516

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