Sync from internal repo (2026/07/07)

[ccampbell-aai]

Copybara sync from the internal repo:

• sample_rate is now optional for Opus encodings (opus, ogg_opus) — the Opus stream is self-describing and the server ignores the value. A pydantic validator (v1- and v2-compatible) still requires it for PCM encodings, where omitting it would silently fall back to the server's 16 kHz default and garble non-16k audio. When omitted, the parameter is not sent on the wire.
• Documentation/comment updates on the Encoding enum and language_codes fields to match.

Resolves the Aikido finding raised on #211 (comments said sample_rate was "optional and ignored" while the field was required — it is now genuinely optional for Opus).

Bumps package version to 0.64.26.