[steps][3/5] Add action interpolation helpers

[sswrk]

Why

Third PR in the @expo/steps local actions stack: small shared helpers for interpolating action-scoped values. Part of ENG-22387.

How

Adds internal actionInterpolation.ts:

• resolveInterpolatedTarget(target, context, legacyResolver): run ${{ }} via the job interpolator, then route legacy ${ steps.* } through a caller-provided resolver (so action scopes can rewrite short ids)
• stringifyInterpolatedResult / stringifyOptionalInterpolatedResult
• containsUnresolvedTemplateReference for deferred template checks

Reuses existing interpolateWithOutputs / BUILD_STEP_OUTPUT_EXPRESSION_REGEXP from template.ts instead of a parallel templateSpans / actionInputs layer. Utilities remain internal (not exported from index.ts).

Test Plan

cd packages/steps && yarn test — actionInterpolation unit tests.

[linear-code]

ENG-22387

[sswrk]

• [steps][3/5] Add action interpolation helpers #4013 👈 (View in Graphite)
• [steps][2/4] Expand composite function references into build steps #3929 : 1 other dependent PR (#4014 )
• [steps][1/4] Add isCompositeFunctionPath and reject local composite functions in custom builds #3956
• [eas-build-job] Add local composite function schema #3928
• main

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[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 3 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (szymonswierk/eng-22387-steps-expand-custom-actions-at-parse-time@b5d4bce). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
packages/steps/src/utils/actionInterpolation.ts	86.37%	3 Missing ⚠️

Additional details and impacted files

@@                                         Coverage Diff                                         @@
##             szymonswierk/eng-22387-steps-expand-custom-actions-at-parse-time    #4013   +/-   ##
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[github-actions]

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