Surface JSON parse detail in streamable-HTTP 400 responses

[ump45nose]

Summary

Surface JSON parse detail in streamable-HTTP 400 responses

Verification

dotnet test tests/ModelContextProtocol.AspNetCore.Tests -f net10.0 --filter 'FullyQualifiedName~StreamableHttpServerConformanceTests' (56 passed); new PostTruncatedJson theory + updated PostMalformedJson red-green verified (3 fail without the fix, 3 pass with it)

Related to #1842

AI assistance disclosure: AI was used to discover this opportunity and draft the change or text. The submission was checked against the prepared artifact and recorded verification evidence.

[ump45nose]

Additional verification context: the change is limited to preserving the existing 400 InvalidRequest behavior while surfacing the caught JsonException message and its line/byte position. The focused net10.0 conformance run passed 56 tests, including the malformed-JSON case and the new truncated-JSON regression coverage tied to #1842.

GitHub currently shows no completed check jobs or reviews for this commit, so the evidence above refers to the recorded local verification only.

AI assistance disclosure: AI was used to discover this opportunity and draft the change or text. The submission was checked against the prepared artifact and recorded verification evidence.

[ump45nose]

Rebased onto current main (cb3e6e87, which brings in #1837's deterministic SEP-2575 status mapping and #1791) and re-verified on the rebased head 30aa2195:

• ModelContextProtocol.AspNetCore.Tests (net10.0, Release): 621 passed, 30 skipped, 0 failed
• ModelContextProtocol.Tests: 2359 passed, 5 skipped, 0 failed
• StreamableHttpServerConformanceTests alone: 56 passed, including PostMalformedJson_Returns400_InvalidRequest_WithNullId and both PostTruncatedJson_Returns400_WithParserDetail cases

The rebase was clean, and the two changes do not overlap: #1837 edited StreamableHttpPostTransport.cs / StreamableHttpServerTransport.cs, while this PR only adds parser detail to the POST-body JsonException catch in StreamableHttpHandler.cs — the 400/InvalidRequest shape and the deterministic status mapping are unchanged.

Force-pushed to the PR branch (contrib/d881e201499f) only; the base branch was not touched, and no upstream branch was modified.

Reproduction note: build/test locally with -p:NuGetAudit=false. Microsoft.Build.Tasks.Git 8.0.0 raises NU1902 on this repository today, and it reproduces on unmodified main, so it is unrelated to this change.

AI assistance disclosure: AI was used to discover this opportunity and draft the change or text. The submission was checked against the prepared artifact and recorded verification evidence.