Voice string format: reconcile ai.languages and TTS provider pages; document :model precedence

[briankwest]

Problem

Two page families document two different voice-string formats with no cross-reference:

• /docs/swml/reference/calling/ai/languages: "Compose the voice string using the <engine id>.<voice id> syntax" (e.g. elevenlabs.rachel), with model documented as a separate property.
• /docs/platform/voice/tts/cartesia and /docs/platform/voice/tts/elevenlabs: colon-suffixed engine.<voice_id>:<model>.

Nothing states whether both work or what happens when the :model suffix and the model property disagree.

Actual behavior (to document)

• The full form is engine.voice_id:model; the engine. prefix and :model suffix are each optional.
• If both the :model suffix and the separate model property are present, the suffix wins.
• Without an engine prefix, the entry's engine property (then the top-level default) applies.

Fix

State the above explicitly on ai.languages, align the TTS pages, and cross-reference in both directions.