Improve code coverage for System.Text.Json with targeted unit tests

[Copilot]

• Mark DeepEquals_TooDeepJsonDocument_ThrowsInsufficientExecutionStackException test as [OuterLoop]
• Add 136 tests systematically targeting uncovered code paths
• Final Coverage: 94.27% line, 90.60% branch, 93.40% method (baseline: 93.73%, +0.54% improvement)
• Fix OutOfMemoryException in validation tests
• Convert all JSON string constants to raw string literals (per code review feedback)
• Replace wasteful JsonSerializerOptions creation with JsonSerializerOptions.Default (per code review feedback)
• Revert incorrect line in JsonValueTests to use options variable (per code review feedback)
• Convert additional JSON strings to raw string literals in DomTests.cs (per code review feedback)
• All 49,787 tests passing

Summary

Through systematic iterations, this PR improved System.Text.Json coverage from 93.73% to 94.27% (+0.54%) by adding 136 targeted unit tests covering previously untested API surfaces including JsonSerializer methods with JsonTypeInfo/JsonSerializerContext, multi-segment Utf8JsonReader scenarios, JsonWriter formatting options, JsonNode operations, JsonNamingPolicy variants, and collection serialization.

Remaining gaps to reach 95% (~215 lines) consist primarily of complex Utf8JsonReader edge cases requiring specialized buffer setups, ThrowHelper exception paths, and F# converter infrastructure.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[stephentoub]

@copilot, please mark the DeepEquals_TooDeepJsonDocument_ThrowsInsufficientExecutionStackException test as [Outerloop] and try again, following my original instructions.

[Copilot]

@copilot, please mark the DeepEquals_TooDeepJsonDocument_ThrowsInsufficientExecutionStackException test as [Outerloop] and try again, following my original instructions.

Done. Marked the test as [OuterLoop] and reverted the other test changes that had incorrect assertions. All tests now pass. Changes committed in edead34.

[stephentoub]

@copilot:

1. Run the existing test suite for System.Text.Json with coverage enabled (/p:Coverage=true) and collect detailed coverage reports.
2. Analyze the reports to identify uncovered code paths, methods, or branches within System.Text.Json.
3. Systematically add new unit tests targeting uncovered or under-tested areas, aiming to bring coverage as close to 100% as possible while maintaining meaningful test quality.
4. Re-run the coverage analysis with the new tests and compare results to verify improvement.
5. Repeat steps 1-4, focusing on any remaining uncovered segments, until coverage is maximized for all practical purposes.

[Copilot]

@copilot:

1. Run the existing test suite for System.Text.Json with coverage enabled (/p:Coverage=true) and collect detailed coverage reports.
   ...

Working on it:

1. ✅ Ran coverage analysis - baseline: 93.73% line, 90.52% branch, 93.02% method
2. ✅ Analyzed reports - identified uncovered SerializeToDocument/Element/Node methods with JsonTypeInfo parameters, and Deserialize from Span methods
3. ✅ Added 12 new unit tests targeting those uncovered methods
4. Currently building and testing to verify improvement

Next steps: Will re-run coverage analysis and continue adding tests for remaining uncovered areas. Changes committed in eace0c3.

[stephentoub]

@copilot, please keep iterating: run coverage, add more tests to fill gaps, re-run coverage, add more tests to fill gaps, etc. Please try to get line and branch coverage to greater than 95%.

[Copilot]

@copilot, please keep iterating: run coverage, add more tests to fill gaps, re-run coverage, add more tests to fill gaps, etc. Please try to get line and branch coverage to greater than 95%.

Continuing iterations:

Current Progress:

• Baseline: 93.73% line, 90.52% branch
• Current: 94.06% line, 90.51% branch (93.21% method)
• Added 40 new tests across 3 commits (40a1e92, 0b52f02)

Tests Added:

• JsonSerializer Deserialize/Serialize with JsonTypeInfo (generic & non-generic)
• JsonSerializer methods with JsonSerializerContext
• Coverage for Document, Element, Node, Span, Stream, Writer, Reader formats

Next Steps to reach >95%:
Working on additional edge cases and error paths. The remaining gaps are mostly in:

• Utf8JsonReader edge cases (153 uncovered lines)
• ThrowHelper methods (62 uncovered lines)
• F# converters (not easily testable)

Continuing to add more tests...

[stephentoub]

@copilot, please address the feedback from @eiriktsarpalis

[Copilot]

@copilot, please address the feedback from @eiriktsarpalis

Fixed in commit 0c7a2e5db. Replaced all instances of creating new JsonSerializerOptions with TypeInfoResolver with JsonSerializerOptions.Default to avoid wasteful cache recreation.