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Summary

Related to #179 — declared response headers from OpenAPI specs were silently discarded because ProvidedApiClientBase.CallAsync returned Task<HttpContent>, throwing away the HttpResponseMessage and all its headers.

Root Cause

// Before: HttpResponseMessage is disposed, headers lost
member _.CallAsync(...) : Task<HttpContent> = task {
    let! response = httpClient.SendAsync(...)
    return response.Content   // headers gone
}

Fix

1. Change CallAsync return type to Task<HttpResponseMessage> — the full response object is now returned.
2. Add LastResponseHeaders property — a mutable instance field on ProvidedApiClientBase that is populated after each CallAsync. Users can read client.LastResponseHeaders to access response headers immediately after any API call.
3. Update OperationCompiler — the generated task { } quotation bodies now call response.Content where content was used directly before.
4. Add collectResponseHeaders helper to RuntimeHelpers for converting HttpResponseHeaders to a flat IReadOnlyDictionary<string, string>.

Usage (after fix)

let! result = client.SomeOperation(...)
let headers = client.LastResponseHeaders
// headers["X-Custom-Header"] etc.

Trade-offs

• CallAsync return type is a technically breaking change for any code that calls it directly (rare; it's a protected base-class method). Generated client code is updated accordingly.
• LastResponseHeaders is per-instance and not thread-safe for concurrent calls on the same client instance — consistent with the general HttpClient usage pattern.

Test Status

• ✅ Build succeeded (dotnet build SwaggerProvider.sln -c Release)
• ✅ 284 tests, 0 failures, 0 errors (SwaggerProvider.Tests.dll)
• ✅ 4 new unit tests added for LastResponseHeaders
• ✅ Format check passed (dotnet fantomas --check)

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