Use System.Text.Json source generation to remove some trim/AOT warnings

[charlesroddie]

Description

Replaces reflection-based JsonSerializer calls with an internal JsonSerializerContext (SqlClientJsonSerializerContext), removing the IL2026/IL3050 warnings they raise under trimming and Native AOT:

• SqlVector<T>.GetString() – serializes ReadOnlyMemory<float> (the constructor already restricts T to float).
• SqlParameter string-to-vector coercion – deserializes float[].
• HostGuardianServiceEnclaveProvider.MakeRequest – deserializes List<byte>.

Trim/AOT warnings with -p:IsAotCompatible=true (unique by location and code):

TFM	Before	After
net8.0	50	44
net9.0	52	46

No public API or behavioral changes; JSON output is unchanged.

Issues

Part of #1947.

Testing

• Built net462, net8.0 and net9.0 with -p:IsAotCompatible=true; none of the touched files emit trim/AOT warnings.
• Existing SqlVector/SqlParameter unit tests (including exact GetString() output) pass on net462, net8.0 and net10.0.
• The string-to-vector and HGS attestation paths are covered only by tests requiring SQL Server / HGS and were not run locally.

🤖 Generated with Claude Code

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Pull request overview

This PR removes trim/NativeAOT warnings by switching JSON (de)serialization from reflection-based JsonSerializer APIs to System.Text.Json source-generated metadata via a new internal SqlClientJsonSerializerContext.

Changes:

• Added SqlClientJsonSerializerContext with [JsonSerializable] entries for the JSON types used.
• Updated SqlVector<T>.GetString() to serialize using source-generated JsonTypeInfo.
• Updated SqlParameter vector coercion and VirtualSecureModeEnclaveProvider.MakeRequest to deserialize using source-generated JsonTypeInfo.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlTypes/SqlVector.cs	Uses source-generated type info for vector JSON serialization
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/VirtualSecureModeEnclaveProvider.cs	Uses source-generated type info for HGS response deserialization
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/SqlParameter.cs	Uses source-generated type info for string-to-vector JSON coercion
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/SqlClientJsonSerializerContext.cs	Introduces the source-generation context and registered serializable types

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[cheenamalhotra]

/azp run

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[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 64.60%. Comparing base (f2310e9) to head (7a2392c).

Files with missing lines	Patch %	Lines
...Data/SqlClient/VirtualSecureModeEnclaveProvider.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4683      +/-   ##
==========================================
- Coverage   66.27%   64.60%   -1.67%     
==========================================
  Files         290      284       -6     
  Lines       45062    68015   +22953     
==========================================
+ Hits        29865    43943   +14078     
- Misses      15197    24072    +8875     

Flag	Coverage Δ
CI-SqlClient	?
PR-SqlClient-Project	64.60% <66.66%> (?)

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