fix(tts): use per-provider input-character caps instead of global 4000 (#13743)

A single global MAX_TEXT_LENGTH = 4000 truncated every TTS provider at
4000 chars, causing long inputs to be silently chopped even though the
underlying APIs allow much more:

  - OpenAI:     4096
  - xAI:        15000
  - MiniMax:    10000
  - ElevenLabs: 5000 / 10000 / 30000 / 40000 (model-aware)
  - Gemini:     ~5000
  - Edge:       ~5000

The schema description also told the model 'Keep under 4000 characters',
which encouraged the agent to self-chunk long briefs into multiple TTS
calls (producing 3 separate audio files instead of one).

New behavior:
  - PROVIDER_MAX_TEXT_LENGTH table + ELEVENLABS_MODEL_MAX_TEXT_LENGTH
    encode the documented per-provider limits.
  - _resolve_max_text_length(provider, cfg) resolves:
      1. tts.<provider>.max_text_length user override
      2. ElevenLabs model_id lookup
      3. provider default
      4. 4000 fallback
  - text_to_speech_tool() and stream_tts_to_speaker() both call the
    resolver; old MAX_TEXT_LENGTH alias kept for back-compat.
  - Schema description no longer hardcodes 4000.

Tests: 27 new unit + E2E tests; all 53 existing TTS tests and 253
voice-command/voice-cli tests still pass.

hermes_cli/config.py

@@ -613,6 +613,10 @@ DEFAULT_CONFIG = {
     },
     
     # Text-to-speech configuration
+    # Each provider supports an optional `max_text_length:` override for the
+    # per-request input-character cap. Omit it to use the provider's documented
+    # limit (OpenAI 4096, xAI 15000, MiniMax 10000, ElevenLabs 5k-40k model-aware,
+    # Gemini 5000, Edge 5000, Mistral 4000, NeuTTS/KittenTTS 2000).
     "tts": {
         "provider": "edge",  # "edge" (free) | "elevenlabs" (premium) | "openai" | "xai" | "minimax" | "mistral" | "neutts" (local)
         "edge": {