Make StackTraceHelper tests runtime-agnostic

[medhatiwari]

Summary

Fixes the remaining 2 failing StackTraceHelper tests from #63927 by making tests runtime-agnostic instead of expecting exact string matches for runtime internals.

Problem

Tests were failing on Mono due to strict string matching against runtime-specific formatting:

• Lambda methods: Expected "lambda_method34" but Mono shows "object.lambda_method34"
• Async state machines: Expected resolved method names but Mono shows state machine internals

Solution

Updated tests to verify essential functionality while accepting different valid runtime behaviors:

• Lambda test: Changed from Assert.StartsWith("lambda_method") to Assert.Contains("lambda_method")
• Async test: Added flexible checks that accept both resolved method names and state machine forms

cc: @giritrivedi

[joperezr]

Thank you so much for your contribution @medhatiwari! Left a small comment that would be good if you could address, but otherwise this is looking really good 😃

[medhatiwari]

Thank you so much for your contribution @medhatiwari! Left a small comment that would be good if you could address, but otherwise this is looking really good 😃

Thanks for reviewing the PR

[giritrivedi]

Can this be merged ?

[giritrivedi]

/azp run

[azure-pipelines[bot]]

Commenter does not have sufficient privileges for PR 64007 in repo dotnet/aspnetcore

[joperezr]

@wtgodbe any idea around the test failures? Unless I'm missing something obvious it seems like they are unrelated to this PR. Would be good to finally get this contribution merged 😃