[Repo Assist] Fix #1419: Add cycle guard for XSD group references to prevent StackOverflow

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Closes #1419

Summary

Adds a cycle guard to XsdInference.fs to prevent a StackOverflowException when parsing XSD schemas whose compiled object graph contains group reference cycles.

When XmlProvider(Schema = "...") is used with a complex schema (such as the MS Office Custom UI schema), parseParticle can recurse infinitely when following XmlSchemaGroupRef.Particle chains that form a loop in the compiled XmlSchemaSet object graph.

Root Cause

In XsdParsing.parseParticle, the XmlSchemaGroupRef case unconditionally recursed into the resolved group's particle:

| :? XmlSchemaGroupRef as grpRef -> parseParticle ctx grpRef.Particle

After .Compile(), the .NET XSD compiler sets grpRef.Particle to the referenced group's XmlSchemaGroupBase. If a complex schema has groups that (directly or transitively through the compiled object graph) reference each other, this creates a cycle: parseParticle(A.Particle) → ... → parseParticle(A.Particle) → ∞.

The existing cycle protection in parseElement (using ctx.Elements cache) handles recursion through element types, but not recursion through group particles themselves.

Fix

Two changes:

XsdInference.fs: Added visitingGroups : HashSet(XmlQualifiedName) to ParsingContext, exposed as member x.VisitingGroups. In parseParticle, the XmlSchemaGroupRef case now checks whether the group is already being visited, returning Empty (an empty particle) to break the cycle:

| :? XmlSchemaGroupRef as grpRef ->
    // Guard against circular group references
    if ctx.VisitingGroups.Contains grpRef.RefName then
        Empty // break the cycle
    else
        ctx.VisitingGroups.Add grpRef.RefName |> ignore
        let result = parseParticle ctx grpRef.Particle
        ctx.VisitingGroups.Remove grpRef.RefName |> ignore
        result

InferenceTests.fs: Added a test verifying that a schema with a recursive content model (a group that, through its element content, references itself) completes without error.

Trade-offs

• Returning Empty for the back-edge of a cycle is the same convention used elsewhere in the code (e.g. XmlSchemaParticle.EmptyParticle falls through to Empty). The inferred type for the cyclic portion will omit the repeated elements — but this is correct behaviour since infinite nesting is not representable in the generated types anyway.
• The fix is minimal and surgical: only the GroupRef case is affected.

Test Status

✅ All 50 InferenceTests pass (including new "circular group references do not cause a stack overflow" test)
⚠️ SignatureTests: 436 pre-existing failures due to infrastructure constraints (network/assembly lookup) — not caused by this PR. These failures were present before this change.

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[github-actions]

✅ Pull request created: #1626