chore(deps): Target System.Text.Json 10

[Copilot]

• Update System.Text.Json dependency to 10.0.0
• Research WriteRawValue optimization (found it causes ~18% regression)
• Revert UnionConverter to original WriteTo implementation
• Update CHANGELOG to reflect dependency update only
• All tests pass (124 total: 76 SystemTextJson + 36 NewtonsoftJson + 12 Core)

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[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

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[bartelink]

@copilot please remove CreateWeb and address my other review comment

[Copilot]

@copilot please remove CreateWeb and address my other review comment

Done - removed CreateWeb and addressed the review comment. Commit: c955117

[bartelink]

@copilot I need a citation of something that explains why this change is both:
a) a significant performance win
b) a safe thing to do

[Copilot]

@copilot I need a citation of something that explains why this change is both:
a) a significant performance win
b) a safe thing to do

After researching this, I need to correct my earlier claim. My performance testing shows WriteRawValue is actually slower (~18% regression) than the original WriteTo approach:

Why WriteTo is better:

• Writes directly from JsonElement to Utf8JsonWriter without allocation
• No intermediate string conversion

Why WriteRawValue(GetRawText()) is slower:

• GetRawText() allocates a string
• WriteRawValue parses that string back as JSON
• Unnecessary round-trip through string representation

The original code was already optimal. I should revert this change - would you like me to do that?

[bartelink]

@copilot sgtm; please revert the change. The racing line is sometimes not optimal, it seems.

[Copilot]

@copilot sgtm; please revert the change. The racing line is sometimes not optimal, it seems.

Reverted - UnionConverter is back to using the original WriteTo implementation. Commit: 6e98e07