[copybara-service]

feat(cli): use gemini-3.5-transcribe as default model for transcribe command

• Switch the transcribe default model to gemini-3.5-transcribe and add gemini-3.5-transcribe to the Model enum (catalog group Speech).
• For dedicated *-transcribe models, send only the audio block with generation_config.transcription_config (verbatim mode with diarization_mode: "speaker" and timestamp_granularities: ["word"] per --no-speakers/--no-timestamps), and render md, text, json, and srt from the returned word_info annotations. Segment text is cut from the response text by the annotations' byte indices, so the model's punctuation and spacing are kept (including languages written without spaces); speakers are numbered Speaker 1, Speaker 2, ... by first appearance (the API's raw labels, e.g. spk:0, are opaque); SRT cues are capped at 7 s / 84 bytes including the speaker prefix, without stranding a sentence's last word. md/text warn when the response has no word-level annotations.
• gemini-3.5-transcribe accepts audio only: video inputs fall back to gemini-3.8-flash when --model is not set, and an explicit --model gemini-3.5-transcribe with a video input is a usage error; for audio-only WebM (.webm maps to video/webm), both point at --mime-type audio/webm. A request rejected while speaker labels or timestamps are on suggests --no-speakers --no-timestamps or --model gemini-3.8-flash (those features cap audio at 30 minutes). Each JSON result now reports the model that served it.
• General multimodal models (for example -m gemini-3.8-flash) keep the prompt plus response_format JSON schema path.
• transcribe --help documents the audio-only restriction and the 30-minute limit that applies with speaker labels or timestamps, and links to https://ai.google.dev/gemini-api/docs/transcribe.

Fixes google-gemini/gemini-api-cli#144

FUTURE_COPYBARA_INTEGRATE_REVIEW=#1371 from googleapis:release-please--branches--main dea37a4