chore(backend): Consolidate v2 placeholder resolution into shared utility. Fixes #12949

[khushiiagrawal]

Description of your changes:

This PR consolidates duplicate placeholder resolution and type stringification logic that existed in both the driver and launcher into a shared utility package, addressing technical debt from PR #12598.

Fixes #12949

What changed:

• Created new backend/src/v2/placeholder/ package with:

  • PbValueToString() - canonical structpb.Value to string conversion
  • ResolveInputParameterPlaceholders() - shared input parameter placeholder resolution
  • InputParameterRe - exported regex pattern for placeholder matching

• Updated backend/src/v2/driver/util.go:

  • Removed duplicate pbValueToString, resolveSinglePlaceholder, resolveInputParameterPlaceholders functions
  • Now calls shared package for common logic
  • Kept driver-specific logic (IfPresent expansion, output placeholder handling)

• Updated backend/src/v2/component/launcher_v2.go:

  • Replaced 25-line inline type conversion switch with single call to shared function
  • Fixed latent bug where null values weren't handled

Why:

• Eliminates code duplication between driver and launcher
• Prevents behavior drift across layers
• Centralizes type conversion and placeholder resolution for easier maintenance
• All existing tests pass with no behavioral changes

Checklist:

• You have signed off your commits
• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.

[google-oss-prow]

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[google-oss-prow]

Hi @khushiiagrawal. Thanks for your PR.

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[khushiiagrawal]

@droctothorpe @hbelmiro PTAL. thanks.

[Copilot]

Pull request overview

This PR centralizes v2 input-parameter placeholder substitution and structpb.Value → string conversion into a new shared backend/src/v2/placeholder package, then updates both the driver and launcher to use that canonical implementation (including proper handling of null values).

Changes:

• Added backend/src/v2/placeholder with shared utilities: PbValueToString, ResolveInputParameterPlaceholders, and InputParameterRe.
• Refactored backend/src/v2/driver/util.go to call the shared placeholder resolver instead of maintaining a duplicate implementation.
• Refactored backend/src/v2/component/launcher_v2.go to use PbValueToString, fixing prior null handling for input parameters.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
backend/src/v2/placeholder/placeholder.go	New shared placeholder/type-stringification utility used by both driver and launcher.
backend/src/v2/placeholder/placeholder_test.go	Unit tests covering value stringification and placeholder substitution scenarios.
backend/src/v2/driver/util.go	Removes duplicated placeholder logic and routes resolution through the shared package.
backend/src/v2/component/launcher_v2.go	Replaces inline structpb.Value kind switching with PbValueToString (incl. null).

[hbelmiro]

@khushiiagrawal can you please rebase?