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: 3bd8206

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 3bd8206

Command	Status	Duration	Result
nx affected --targets=test:sherif,test:knip,tes...	✅ Succeeded	46s	View ↗
nx run-many --target=build --exclude=examples/**	✅ Succeeded	<1s	View ↗

---

☁️ Nx Cloud last updated this comment at 2026-03-09 08:05:41 UTC

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commit: 572d59f

[sentry]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.24%. Comparing base (6892ed0) to head (3bd8206).
⚠️ Report is 150 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2012      +/-   ##
==========================================
- Coverage   90.35%   90.24%   -0.11%     
==========================================
  Files          38       49      +11     
  Lines        1752     2010     +258     
  Branches      444      520      +76     
==========================================
+ Hits         1583     1814     +231     
- Misses        149      176      +27     
  Partials       20       20              

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