Fix generic async result double wrapping

[DavidObando]

Summary

• preserve authoritative generic method arguments through bound-tree rewrites and async spilling
• infer fallback MethodSpec arguments from the declared async result by removing exactly one observable Task/ValueTask wrapper
• cover open/closed generic runtime + ILVerify, non-generic, imported, nested Task, tuple, and non-async negative controls

Verification

• baseline exact repro: 2 StackUnexpected findings (Task<Task<T0>> and Task<Task<int32>>)
• fixed exact repro: ILVerify clean and runtime output 42
• targeted Core.Tests: 2 passed
• targeted Compiler.Tests: 4 passed
• full Core.Tests: 6,693 passed, 1 skipped
• full Compiler.Tests: 3,405 passed (after building the required Gsharp.Extensions prerequisite)

Deferred work

• None

Fixes #2751