fix(expect): name matcher in toMatchObject failure message

[fibibot]

Summary

expect(received).toMatchObject(expected) previously failed with
AssertionError: Values are not equal., and the .not.toMatchObject
branch produced Expected actual: ... not to be: .... Both are
misleading because toMatchObject is a superset check, not strict
equality.

The triggerError closure in expect/_matchers.ts reused
buildEqualErrorMessage / buildNotEqualErrorMessage from
expect/_build_message.ts. This change adds two matcher-specific
builders, buildToMatchObjectErrorMessage and
buildNotToMatchObjectErrorMessage, that emit Jest-style headers:

expect(received).toMatchObject(expected)

<diff>

expect(received).not.toMatchObject(expected)

Expected: not <expected>
Received: <received>

Both branches of toMatchObject are routed through the new builders.
The diff content (already trimmed to the subset by PR #7078) is
preserved.

Test plan

• Updated expect/_to_match_object_test.ts "throws the correct
  error messages" case to assert on the new matcher-named prefix
  and explicitly reject the previous Values are not equal /
  not to be phrasings.
• deno fmt --check passes for expect/.
• deno lint passes for expect/.
• deno test -A expect/ passes (167 passed, 0 failed).
• Reproduced the original issue locally; failure message now reads
  expect(received).toMatchObject(expected) instead of Values are not equal..

Closes #6999
Closes bartlomieju/orchid-inbox#90

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[codecov]

Codecov Report

❌ Patch coverage is 87.87879% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.60%. Comparing base (5ea9159) to head (d0288ca).

Files with missing lines	Patch %	Lines
expect/_build_message.ts	85.18%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7142      +/-   ##
==========================================
- Coverage   94.61%   94.60%   -0.01%     
==========================================
  Files         634      634              
  Lines       51830    51860      +30     
  Branches     9341     9345       +4     
==========================================
+ Hits        49037    49063      +26     
  Misses       2218     2218              
- Partials      575      579       +4     

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[bartlomieju]

Thanks for the fix — the message is much clearer now.

One cleanup before merge: buildToMatchObjectErrorMessage is essentially buildEqualErrorMessage with a different header line, and buildNotToMatchObjectErrorMessage is buildNotEqualErrorMessage with a different header. Rather than duplicating the diff/format logic in _build_message.ts, please parameterize the existing builders with an optional header, e.g.:

export function buildEqualErrorMessage<T>(
  actual: T,
  expected: T,
  options: EqualErrorMessageOptions & { header?: string } = {},
): string {
  const { formatter = format, msg, header = "Values are not equal." } = options;
  // ...
  let message = `${msgPrefix}${header}`;
  // ...
}

Then toMatchObject can call the existing functions with the matcher-named header. That way the diff path stays in one place and the two builders don't drift if one is later updated.

Also: one optional nice-to-have in the test — assert that the diff body is actually present in the positive case (e.g. assertMatch(e.message, /-\s+a: 2/)) so a future refactor that accidentally strips the diff gets caught.