Fix flaky OpenAiSttService SSE test (deterministic progress)

[niksedk]

TranscribeAsync_SseStream_AccumulatesDeltasAndReportsDoneSegments intermittently failed in the full (parallel) test run while always passing in isolation.

Root cause (test-only)

The service reports deltas/segments synchronously via IProgress<T>.Report while parsing the SSE stream (before TranscribeAsync returns). The test used the BCL Progress<T>, which doesn't run callbacks inline — it posts them to the captured SynchronizationContext, or to the thread pool if none. The test then polled WaitForAsync(..., 2000ms) and asserted on plain List<>s. Under full-suite xUnit parallelism this flaked two ways:

• posted callbacks delayed past the 2 s deadline → incomplete list → ordered-equality assert fails;
• with no sync context, callbacks ran on thread-pool threads, racing on the non-thread-safe List<>.

Production Progress<T> (marshaling to the UI thread) is correct — this was purely a test artifact, and only this one test used the pattern.

Fix

Use a tiny synchronous IProgress<T> stub so callbacks run inline on the parse thread; the lists are complete and ordered when TranscribeAsync returns, so assertions run directly and the polling WaitForAsync helper is removed.

Verification

Ran the full UI suite 3×: 481/481 each time (previously intermittently 480/481), and the class 5× in isolation.

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