fix(nemo): extract Hypothesis.text for TDT/RNNT ASR models (#10012)

* fix(nemo): extract Hypothesis.text for TDT/RNNT ASR models

CTC models (e.g. Whisper) return List[str] from transcribe(), but
TDT/RNNT models (e.g. parakeet-tdt-0.6b-v3) return List[Hypothesis]
where the decoded text lives in the Hypothesis.text attribute.

Previously, results[0] was assigned directly to the protobuf string
field, causing silent empty output for non-CTC models.

Now checks the return type and extracts .text from Hypothesis objects,
with a safe fallback via getattr().

* refactor: simplify Hypothesis text extraction per Copilot review

Use single getattr() call instead of hasattr() + double access,
and return empty string for unknown types instead of str(result)
to avoid leaking internal repr to clients.