fix(audio): replace unreachable fallback in primaryRateOrder with Q_UNREACHABLE (#3432)

[M7HNF-Ian]

The trailing return {internalRate} after both direction/OS switch statements
in primaryRateOrder() is dead code — every TargetOs value is covered in
both the Output and Input branches. It silently masks future omissions: adding
a new TargetOs enum value without updating a switch would compile cleanly
and return the wrong rate list with no diagnostic.

Replace with Q_UNREACHABLE() so the invariant ("every TargetOs value is
handled in both directions") is enforced by the compiler/runtime rather than
papered over with a silent default.

Testing

All 25 audio_format_negotiation_test checks pass (built and run locally on
macOS against the modified AudioFormatNegotiator.cpp).

Closes #3432

[aethersdr-agent]

Thanks for the cleanup, @M7HNF-Ian — the rationale is exactly right. The trailing return {internalRate} was a silent landmine: adding a fourth TargetOs value would compile cleanly and quietly return only the internal rate, breaking the per-OS ladder in a way that no test or warning would catch. Turning it into a hard invariant via Q_UNREACHABLE() is the right move.

A few notes for awareness (none blocking):

• First Q_UNREACHABLE() in this codebase. I grepped and there are no existing uses. The neighbour toString(TargetOs) (AudioFormatNegotiator.cpp:228) and toString(ResamplerKind) (line 218) still use silent return "?" defaults. That's fine — the divergence is justified because primaryRateOrder() feeds a real negotiation path while the toString defaults are debug-only — but worth flagging in case the project later wants to standardise on one pattern.

• Q_UNREACHABLE() vs Q_UNREACHABLE_RETURN(...): Qt 6 ships both. Plain Q_UNREACHABLE() should be sufficient with Clang/GCC/MSVC since it expands to __builtin_unreachable() / __assume(0), but if any CI compiler is older or treats the macro defensively, you may see a "control reaches end of non-void function" warning. If that surfaces, swap to Q_UNREACHABLE_RETURN({}) to suppress it without weakening the invariant. Not needed pre-emptively.

• Test coverage: confirming locally that all 25 audio_format_negotiation_test checks pass is exactly the right validation — the function is pure-policy by design (per the file header comment), so unit tests are the appropriate gate.

LGTM as a defensive correctness improvement. Nice catch.

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