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>

run_agent.py

@@ -691,9 +691,14 @@ class AIAgent:
             self.api_mode = api_mode
         elif self.provider == "openai-codex":
             self.api_mode = "codex_responses"
+        elif self.provider == "xai":
+            self.api_mode = "codex_responses"
         elif (provider_name is None) and "chatgpt.com/backend-api/codex" in self._base_url_lower:
             self.api_mode = "codex_responses"
             self.provider = "openai-codex"
+        elif (provider_name is None) and "api.x.ai" in self._base_url_lower:
+            self.api_mode = "codex_responses"
+            self.provider = "xai"
         elif self.provider == "anthropic" or (provider_name is None and "api.anthropic.com" in self._base_url_lower):
             self.api_mode = "anthropic_messages"
             self.provider = "anthropic"
@@ -4032,6 +4037,7 @@ class AIAgent:
             "model", "instructions", "input", "tools", "store",
             "reasoning", "include", "max_output_tokens", "temperature",
             "tool_choice", "parallel_tool_calls", "prompt_cache_key", "service_tier",
+            "extra_headers",
         }
         normalized: Dict[str, Any] = {
             "model": model,
@@ -4067,6 +4073,20 @@ class AIAgent:
             if val is not None:
                 normalized[passthrough_key] = val
 
+        extra_headers = api_kwargs.get("extra_headers")
+        if extra_headers is not None:
+            if not isinstance(extra_headers, dict):
+                raise ValueError("Codex Responses request 'extra_headers' must be an object.")
+            normalized_headers: Dict[str, str] = {}
+            for key, value in extra_headers.items():
+                if not isinstance(key, str) or not key.strip():
+                    raise ValueError("Codex Responses request 'extra_headers' keys must be non-empty strings.")
+                if value is None:
+                    continue
+                normalized_headers[key.strip()] = str(value)
+            if normalized_headers:
+                normalized["extra_headers"] = normalized_headers
+
         if allow_stream:
             stream = api_kwargs.get("stream")
             if stream is not None and stream is not True:
@@ -6504,7 +6524,12 @@ class AIAgent:
             if not is_github_responses:
                 kwargs["prompt_cache_key"] = self.session_id
 
-            if reasoning_enabled:
+            is_xai_responses = self.provider == "xai" or "api.x.ai" in (self.base_url or "").lower()
+
+            if reasoning_enabled and is_xai_responses:
+                # xAI reasons automatically — no effort param, just include encrypted content
+                kwargs["include"] = ["reasoning.encrypted_content"]
+            elif reasoning_enabled:
                 if is_github_responses:
                     # Copilot's Responses route advertises reasoning-effort support,
                     # but not OpenAI-specific prompt cache or encrypted reasoning
@@ -6515,7 +6540,7 @@ class AIAgent:
                 else:
                     kwargs["reasoning"] = {"effort": reasoning_effort, "summary": "auto"}
                     kwargs["include"] = ["reasoning.encrypted_content"]
-            elif not is_github_responses:
+            elif not is_github_responses and not is_xai_responses:
                 kwargs["include"] = []
 
             if self.request_overrides:
@@ -6524,6 +6549,9 @@ class AIAgent:
             if self.max_tokens is not None and not is_codex_backend:
                 kwargs["max_output_tokens"] = self.max_tokens
 
+            if is_xai_responses and getattr(self, "session_id", None):
+                kwargs["extra_headers"] = {"x-grok-conv-id": self.session_id}
+
             return kwargs
 
         sanitized_messages = api_messages
@@ -6706,12 +6734,6 @@ class AIAgent:
         if extra_body:
             api_kwargs["extra_body"] = extra_body
 
-        # xAI prompt caching: send x-grok-conv-id header to route requests
-        # to the same server, maximizing automatic cache hits.
-        # https://docs.x.ai/developers/advanced-api-usage/prompt-caching
-        if "x.ai" in self._base_url_lower and hasattr(self, "session_id") and self.session_id:
-            api_kwargs["extra_headers"] = {"x-grok-conv-id": self.session_id}
-
         # Priority Processing / generic request overrides (e.g. service_tier).
         # Applied last so overrides win over any defaults set above.
         if self.request_overrides: