feat(xai): upgrade to Responses API, add TTS provider

Cherry-picked and trimmed from PR #10600 by Jaaneek.

- Switch xAI transport from openai_chat to codex_responses (Responses API)
- Add codex_responses detection for xAI in all runtime_provider resolution paths
- Add xAI api_mode detection in AIAgent.__init__ (provider name + URL auto-detect)
- Add extra_headers passthrough for codex_responses requests
- Add x-grok-conv-id session header for xAI prompt caching
- Add xAI reasoning support (encrypted_content include, no effort param)
- Move x-grok-conv-id from chat_completions path to codex_responses path
- Add xAI TTS provider (dedicated /v1/tts endpoint with Opus conversion)
- Add xAI provider aliases (grok, x-ai, x.ai) across auth, models, providers, auxiliary
- Trim xAI model list to agentic models (grok-4.20-reasoning, grok-4-1-fast-reasoning)
- Add XAI_API_KEY/XAI_BASE_URL to OPTIONAL_ENV_VARS
- Add xAI TTS config section, setup wizard entry, tools_config provider option
- Add shared xai_http.py helper for User-Agent string

Co-authored-by: Jaaneek <Jaaneek@users.noreply.github.com>

hermes_cli/models.py

@@ -145,17 +145,8 @@ _PROVIDER_MODELS: dict[str, list[str]] = {
         "glm-4.5-flash",
     ],
     "xai": [
-        "grok-4.20-0309-reasoning",
-        "grok-4.20-0309-non-reasoning",
-        "grok-4.20-multi-agent-0309",
+        "grok-4.20-reasoning",
         "grok-4-1-fast-reasoning",
-        "grok-4-1-fast-non-reasoning",
-        "grok-4-fast-reasoning",
-        "grok-4-fast-non-reasoning",
-        "grok-4-0709",
-        "grok-code-fast-1",
-        "grok-3",
-        "grok-3-mini",
     ],
     "kimi-coding": [
         "kimi-for-coding",