fix(expect): toMatchObject failure headline describes a subset match

[LeSingh1]

Fixes #6999.

expect(x).toMatchObject(y) is a subset check, not equality, so failing with Values are not equal. misleads the reader into thinking the test is asking for strict equality. From the issue's repro:

error: AssertionError: Values are not equal.

    [Diff] Actual / Expected

    {
      position: {
+       x: Any { inverse: false, value: [Function: Number] },
+       y: Any { inverse: false, value: [Function: Number] },
-       x: 0,
-       y: "string",
      },
    }

After:

error: AssertionError: Object does not match the expected pattern.

    [Diff] Actual / Expected

    {
      position: {
        x: 0,
-       y: "string",
+       y: 0,
      },
    }

Implementation: buildEqualErrorMessage now accepts an optional summary override on its options. Default is "Values are not equal." so toEqual and the other equality matchers are unchanged. toMatchObject passes summary: "Object does not match the expected pattern." so the headline matches what it actually checks. The diff body and the customMessage prefix are untouched.

Test added in expect/_to_match_object_test.ts pins the new headline and verifies the old one is no longer present. The full --filter toMatchObject run passes (7/7).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.57%. Comparing base (cdf74a8) to head (b3909d8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7168      +/-   ##
==========================================
- Coverage   94.57%   94.57%   -0.01%     
==========================================
  Files         636      636              
  Lines       52142    52145       +3     
  Branches     9401     9401              
==========================================
+ Hits        49315    49317       +2     
  Misses       2249     2249              
- Partials      578      579       +1     

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