Add WithBody(object) overload to RequestMockBuilder for JSON body matching

[Copilot]

• Add WithBody(object body) overload to RequestMockBuilder that serializes the object to JSON and matches using JSON equivalence
• Add tests to HttpMockSpecs.cs for the new method (success case + error message case)
• Update API verification files for net8.0 and net472 (via AcceptApiChanges)
• Update legacy API verification files for net47, netstandard2.0, netstandard2.1 for consistency
• Remove the "invalid JSON" sentence from WithBody(object) XML remarks
• Add WithBody(object) usage example to website/docs/advanced.md

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add WithBody to the RequestMockBuilder that takes an object that will be serialized to JSON</issue_title>
<issue_description>This should enable scenarios like

httpMock
            .ForPatch().WithPath($"IncomeRelationships/{contact.PrimaryIncomeRelationship.Id}")
            .WithBody(new
            {
                EntityKey = "TheRuleKey",
                RepresentativeId = representativeToAssign.ToString()
            })
            .RespondsWithStatus(HttpStatusCode.NoContent);
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Add WithBody to the RequestMockBuilder that takes an object that will be serialized to JSON #79

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[github-actions]

Test Results

  3 files  ±0    3 suites  ±0   6s ⏱️ -1s
129 tests +2  129 ✅ +2  0 💤 ±0  0 ❌ ±0 
252 runs  +4  252 ✅ +4  0 💤 ±0  0 ❌ ±0 

Results for commit e660a11. ± Comparison against base commit 4f5b817.

♻️ This comment has been updated with latest results.

[coveralls]

Pull Request Test Coverage Report for Build 22917510352

Details

• 5 of 7 (71.43%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.09%) to 83.371%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
Mockly/RequestMockBuilder.cs	5	7	71.43%

Totals
Change from base Build 22618149662:	-0.09%
Covered Lines:	1198
Relevant Lines:	1371

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💛 - Coveralls