test(form-core): improve mergeForm coverage to 100%

[Kyujenius]

🎯 Changes

This PR improves test coverage for mergeForm.ts from 87.09% to 100% by adding comprehensive tests for previously uncovered edge cases.

Coverage Improvement

Before:

• Uncovered lines: 17, 19, 105-106

After:

• All lines covered

Type System Note

The mergeForm tests use as any type assertion due to FormApi's 12 generic parameters where TSubmitMeta defaults to never, creating incompatibility with mergeForm's any constraint. This is a type system limitation only - runtime behavior is correct. In production usage via useTransform, this receives AnyFormApi and avoids the issue.

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

This PR only adds tests without modifying any implementation code or public APIs. No changeset is required.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 2baa8f4

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

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[nx-cloud]

View your CI Pipeline Execution ↗ for commit 2baa8f4

Command	Status	Duration	Result
nx affected --targets=test:sherif,test:knip,tes...	✅ Succeeded	43s	View ↗
nx run-many --target=build --exclude=examples/**	✅ Succeeded	2s	View ↗

---

☁️ Nx Cloud last updated this comment at 2026-02-02 14:36:22 UTC

[pkg-pr-new]

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commit: 2baa8f4

[sentry]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.85%. Comparing base (6892ed0) to head (2baa8f4).
⚠️ Report is 128 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2012      +/-   ##
==========================================
- Coverage   90.35%   89.85%   -0.50%     
==========================================
  Files          38       48      +10     
  Lines        1752     1952     +200     
  Branches      444      491      +47     
==========================================
+ Hits         1583     1754     +171     
- Misses        149      177      +28     
- Partials       20       21       +1     

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