Retain rejected output and repair failure context

[robgruen]

Summary

Companion to microsoft/TypeChat#457, which addresses microsoft/TypeChat#116 for TypeScript and Python.

• Add shared FailureInfo to failed Result<T> instances and TypeChatException, keeping rejected output separate from the typed success Value.
• Retain parsed rejected JSON as an independent JsonElement, raw validator InputText, and original model ResponseText. Honor configured JSON depth, comment, and trailing-comma rules rather than reparsing rejected input with looser settings.
• Retain rejected constraint instances and program inputs, including when validators are used directly.
• Preserve explicit null/falsy values, custom failure metadata and subclasses, and the latest useful candidate when a repair returns malformed output or encounters an expected transport failure.
• Enrich HTTP, I/O, response JSON-decoding, and nested TypeChat exceptions during repair through Exception.Data[FailureInfo.ExceptionDataKey], then rethrow the original exception. Initial transport failures, cancellation, and programming exceptions remain unchanged.
• Keep typed success values, existing error messages, configurable repair limits, diagnostic events, and the bounded repair prompt window unchanged. No full attempt history is retained.
• Document the additive API and sensitive-output handling. This is a standalone PR against main, with no unmerged PR dependencies.

API

var failure = Result<MyType>.Error(message).WithFailure(new FailureInfo
{
    Value = rejected,
    ResponseText = originalResponse
});

FailureInfo.HasValue distinguishes unavailable data from an explicit null, false, zero, or empty string. WithFailure returns a shallow copy and preserves existing payloads and custom metadata rather than mutating a reusable validator result.

The existing throwing translation API is preserved. Callers can inspect TypeChatException.Failure; Response continues to expose the original response. Candidate data may precede the latest response text when a repair cannot be parsed.

Validation

• All 206 .NET unit tests passed; core netstandard2.0 and referencing projects built successfully.
• Regressions cover JSON/constraint/program validators, value-type result conversion, payload availability, cloned JSON lifetime, custom subclasses, successful/failed/malformed/multiple repairs, transport failures, cancellation, and cross-request isolation.
• Exact parser-option regressions cover MaxDepth = 1, disallowed comments, and disallowed trailing commas.
• Scoped dotnet format whitespace --verify-no-changes passed.
• git diff --check passed.

SourceLink audit remediation

Commit 0f18283 removes the redundant SourceLink.GitHub 8.0.0 package reference that overrode SDK-integrated SourceLink and pulled in the audited Microsoft.Build.Tasks.Git 8.0.0 package (GHSA-23fw-v26w-5fgq). Microsoft recommends serviced SDKs because there is no patched 8.0 NuGet release. The repository now uses SourceLink supplied by .NET SDK 8+, documents the fixed SDK requirement, and keeps transitive NuGet auditing enabled without suppressions.

Validation of the remediation:

• Full solution Release build with /warnaserror: 0 warnings and 0 errors.
• All 206 Release unit tests passed on rerun.
• The vulnerable SourceLink/Build.Tasks.Git NuGet chain is absent from all 19 restored projects.
• Actual SourceLink custom-debug records and GitHub commit mappings remain embedded in all five library PDBs.

The two prior spell-check findings were corrected in d1221af. All PR checks now pass on the remediation commit, including both .NET 8 CI build/test matrix jobs, CodeQL, and TypeChat.NET CI.