Fix generated handler namespace resolution

[sean0407]

Issue #, if available: #2586

Description of changes:

Qualify a [FromServices] parameter type with global:: when a namespace member beneath the handler's containing namespace can shadow the type's root namespace. This prevents the generated handler from binding the service type to a sibling namespace.

Summary

• Detect a shadowing namespace member for [FromServices] parameter types.
• Use Roslyn's fully qualified display name only when the namespace is shadowed; preserve existing generated output otherwise.
• Add a regression test that compiles a generated handler with an Acme.Common namespace shadowing a global Common service type.

Evidence

• Baseline regression reproduction failed with CS0234 resolving Common.Lambda.Queue.Processor as Acme.Common.Lambda.Queue.Processor.
• Focused regression test passed: dotnet test Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/Amazon.Lambda.Annotations.SourceGenerators.Tests.csproj --framework net8.0 -p:TargetFrameworks=net8.0 --no-restore --filter FullyQualifiedName~FromServicesType_IsResolvedFromGlobalNamespaceWhenNestedNamespaceShadowsIt (1 passed).
• Existing AuthorizerBindingFailureTests passed (11 passed).
• git diff --check passed.

Merge Danger

Door: Generated service type names derive from Roslyn symbols and are only globally qualified when the handler namespace contains a shadowing member with the same root name.

Blast Radius: This affects generated dependency resolution for [FromServices] parameters in the conflicting namespace case. Existing type-name comparisons for Lambda context and generated request types are preserved.

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