perf(generator): return the request task instead of awaiting it

[glennawatson]

What kind of change does this PR introduce?

Performance optimisation in the source generator.

What is the new behavior?

No generated Refit method carries an async state machine; every method hands its task straight back to the caller.

• The reflection fallback path emits a plain task-returning method. A method the generator cannot build inline now emits return (Task<T>)refitFunc(this.Client, args); rather than return await ((Task<T>)refitFunc(...)).ConfigureAwait(false); inside an async method. The awaited form allocated a state machine per call whose only job was to re-wrap the task the request builder had already produced.
• Both generated request paths now agree. Inline request building already returned its task, so the emitted shape no longer depends on whether a method happens to be inline-eligible.
• The emitters lost the async plumbing that had become constant. The return-shape helper now yields a statement prefix instead of an async flag plus a configure-await suffix, and the method-opening and return-statement emitters drop the parameters that fell out with it.

What is the current behavior?

Task-returning methods on the reflection fallback path are emitted async and await the request builder's task before returning it, allocating a second state machine on every call to that method.

What might this PR break?

• Exceptions raised before dispatch now surface synchronously on the fallback path. A missing request builder, or a failure inside BuildRestResultFuncForMethod, previously surfaced as a faulted task; it now throws from the call itself. Callers that invoke a fallback method without awaiting it, and rely on catching around the await, need to catch around the call. Inline-built methods already behaved this way.
• Stack traces no longer contain the generated method frame for fallback methods. The failed call is still identifiable from the exception: ApiExceptionBase.HttpMethod, .Uri, and the Refit.MethodName / Refit.RelativePathTemplate request options.

Checklist

• I have read the Contribute guide
• Tests have been added or updated (for bug fixes / features)
• Docs have been added or updated (for bug fixes / features)
• Changes target the main branch
• PR title follows Conventional Commits

Additional information

The fallback emission shape had no test coverage, so the change would have gone unnoticed by the suite. SwitchOnFallbackReturnsTheRequestBuilderTaskWithoutAwaiting now pins it.

A sweep of the runtime for the same pattern found nothing to remove. Every remaining async method needs its state machine: a using or try scope that has to outlive the await, sequential awaits, work after the await, or an async iterator. Two methods forward a single call but convert ValueTask<T> to Task<T> across the boundary, where eliding would mean an unconditional AsTask() allocation in place of a state-machine box that is only allocated when the await actually suspends.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.91%. Comparing base (51368ea) to head (e5ea23f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2302      +/-   ##
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- Coverage   99.91%   99.91%   -0.01%     
==========================================
  Files         192      192              
  Lines       10031    10029       -2     
  Branches     1925     1924       -1     
==========================================
- Hits        10022    10020       -2     
  Partials        9        9              

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