Harden handling for generic type arguments in source generator

[captainsafia]

Closes #61035.

[captainsafia]

@BrennanConroy @martincostello I talked with both of you online about the notion of switching to a string-based key and what the key would be. Turns out that MetadataName isn't full-proof since it's only defined/exposed for Types so we still have to figure out a mapping for MethodInfo-based values.

I dusted off an old idea around reusing the DocumentationIds that exist in the XML doc comment to support this. We know emit a helper method that emits a documentation ID from PropertyInfo, Type, or MethodInfo. I used Copilot to iterate on this API and it seems to provide coverage for the test cases we have here.

Worst case scenario: the failure mode here is a lot nicer since we won't emit any compiler errors for invalid keys. In the event that happens, the implementation will no-op and fail to apply a doc comment but it shouldn't impact users builds.

[martincostello]

I would err on the side of caution here and format the arity explicitly with CultureInfo.InvariantCulture so that users on systems configured for non-English won't potentially get incorrect number formatting (e.g. scenario of 1000 becoming 1,000 or 1.000). Might be some other occurrences of this elsewhere too.

[martincostello]