Fix NativeAOT namespace lookup for nested generic types

[Copilot]

NativeAOT returned null for Type.Namespace on nested generic types, despite their namespace-qualified FullName.

• Metadata: Stop emitting namespace definitions for nested types and document the schema invariant.
• Reflection: Resolve namespaces through enclosing types; exclude nested types from top-level well-known attribute matching.
• Coverage: Add cases for nested generic definitions and constructions, global-namespace nesting, and nested attribute lookalikes.

Fixes #135106

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[Copilot]

Copilot review overview

🔵 Needs a closer look

The approach appears consistent, but changing NativeAOT’s metadata schema and every major consumer has a broad binary-format blast radius requiring maintainer review.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes NativeAOT namespace resolution for nested types by encoding their structural parent and traversing enclosing types during reflection.

Changes:

• Unifies namespace/enclosing-type metadata.
• Updates all NativeFormat consumers.
• Adds nested generic, global namespace, and attribute-lookalike tests.

File	Description
TypeInfoTests.cs	Adds regression coverage.
NativeFormatReaderGen.cs	Reads the unified parent handle.
MetadataTypeHashingAlgorithms.cs	Hashes types by structural parent.
SchemaDef.cs	Defines the new schema invariant.
NativeMetadataWriter.cs	Formats unified parent records.
NativeFormatWriterGen.cs	Writes the unified parent handle.
Transform.Type.cs	Emits namespace or enclosing type.
MetadataNameExtensions.cs	Traverses structural parents.
StackTraceMetadata.cs	Resolves assembly through parents.
MethodNameFormatter.cs	Formats nested type names.
NativeFormatRuntimeNamedTypeInfo.cs	Resolves inherited namespaces.
TypeResolver.NativeFormat.cs	Uses the renamed parent handle.
MetadataReaderExtensions.NativeFormat.cs	Excludes nested attribute lookalikes.

[jkotas]

/azp run runtime-nativeaot-outerloop

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[jkotas]

/azp run runtime-nativeaot-outerloop

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[jkotas]

/azp run runtime-nativeaot-outerloop

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[jkotas]

/ba-g known issue #135096