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

[dotnet-policy-service]

Thanks for your PR, @@medhatiwari. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.