Feat: Add audio transcription example to the demo

[GtechGovind]

What

I replaced the Transcribe placeholder in the demo with an audio-file transcription example. It shows the selected file and size, offers model/language/prompt options, and displays a selectable transcript with a copy action.

Why

Closes #49. The image example is already implemented; this completes the remaining audio-transcription example through the existing SDK.

Affected Areas

• Misc → Audio → Transcribe uses the currently configured client instead of creating a separate default client. Custom providers require an explicit model ID and an audio/transcriptions-compatible endpoint.
• File loading runs off the main thread, balances security-scoped access, and rejects unsupported extensions, empty files, folders, and files over 25 MB. Selecting a file does not upload it.
• Requests have progress, cancellation, errors and retry. Cancelled or superseded work cannot overwrite newer state; duplicate submissions are ignored.
• DemoChat tests cover file validation, exact query construction, success/error/retry, cancellation, late results, and configured-client replacement. Demo instructions and release-check cases are updated.

More Info

Validation:

• swift test: 225 XCTest tests and 33 Swift Testing tests passed.
• SDK iOS Simulator tests: the same 258 tests passed.
• DemoChat iOS Simulator tests: all 54 tests passed, including 20 new test functions with parameterized cases.
• Demo generic iOS Simulator build passed.
• swift package diagnose-api-breaking-changes 0.5.2: no breaking changes.
• git diff --check passed.

Tests use local files and injected responses, including an intercepted SDK session; no live API requests were made. Interactive Files-provider and clipboard verification remains a manual follow-up. This example does not add recording, streaming, translation, or public SDK API changes.