feat: provider modules — ProviderProfile ABC, 33 providers, fetch_models, transport single-path

Introduces providers/ package — single source of truth for every
inference provider. Adding a simple api-key provider now requires one
providers/<name>.py file with zero edits anywhere else.

What this PR ships:
- providers/ package (ProviderProfile ABC + 33 profiles across 4 api_modes)
- ProviderProfile declarative fields: name, api_mode, aliases, display_name,
  env_vars, base_url, models_url, auth_type, fallback_models, hostname,
  default_headers, fixed_temperature, default_max_tokens, default_aux_model
- 4 overridable hooks: prepare_messages, build_extra_body,
  build_api_kwargs_extras, fetch_models
- chat_completions.build_kwargs: profile path via _build_kwargs_from_profile,
  legacy flag path retained for lmstudio/tencent-tokenhub (which have
  session-aware reasoning probing that doesn't map cleanly to hooks yet)
- run_agent.py: profile path for all registered providers; legacy path
  variable scoping fixed (all flags defined before branching)
- Auto-wires: auth.PROVIDER_REGISTRY, models.CANONICAL_PROVIDERS,
  doctor health checks, config.OPTIONAL_ENV_VARS, model_metadata._URL_TO_PROVIDER
- GeminiProfile: thinking_config translation (native + openai-compat nested)
- New tests/providers/ (79 tests covering profile declarations, transport
  parity, hook overrides, e2e kwargs assembly)

Deltas vs original PR (salvaged onto current main):
- Added profiles: alibaba-coding-plan, azure-foundry, minimax-oauth
  (were added to main since original PR)
- Skipped profiles: lmstudio, tencent-tokenhub stay on legacy path (their
  reasoning_effort probing has no clean hook equivalent yet)
- Removed lmstudio alias from custom profile (it's a separate provider now)
- Skipped openrouter/custom from PROVIDER_REGISTRY auto-extension
  (resolve_provider special-cases them; adding breaks runtime resolution)
- runtime_provider: profile.api_mode only as fallback when URL detection
  finds nothing (was breaking minimax /v1 override)
- Preserved main's legacy-path improvements: deepseek reasoning_content
  preserve, gemini Gemma skip, OpenRouter response caching, Anthropic 1M
  beta recovery, etc.
- Kept agent/copilot_acp_client.py in place (rejected PR's relocation —
  main has 7 fixes landed since; relocation would revert them)
- _API_KEY_PROVIDER_AUX_MODELS alias kept for backward compat with existing
  test imports

Co-authored-by: kshitijk4poor <82637225+kshitijk4poor@users.noreply.github.com>
Closes #14418

providers/gemini.py

@@ -0,0 +1,72 @@
+"""Google Gemini provider profiles.
+
+gemini:            Google AI Studio (API key) — uses GeminiNativeClient
+google-gemini-cli: Google Cloud Code Assist (OAuth) — uses GeminiCloudCodeClient
+
+Both report api_mode="chat_completions" but use custom native clients
+that bypass the standard OpenAI transport. The profile captures auth
+and endpoint metadata for auth.py / runtime_provider.py migration, and
+carries the thinking_config translation hook so the transport's profile
+path produces the same extra_body shape the legacy flag path did.
+"""
+
+from typing import Any
+
+from providers import register_provider
+from providers.base import ProviderProfile
+
+
+class GeminiProfile(ProviderProfile):
+    """Gemini — translate reasoning_config to thinking_config in extra_body."""
+
+    def build_extra_body(
+        self, *, session_id: str | None = None, **context: Any
+    ) -> dict[str, Any]:
+        """Emit extra_body.thinking_config (native) or extra_body.extra_body.google.thinking_config
+        (OpenAI-compat /openai subpath), mirroring the legacy path's behavior.
+        """
+        from agent.transports.chat_completions import (
+            _build_gemini_thinking_config,
+            _is_gemini_openai_compat_base_url,
+            _snake_case_gemini_thinking_config,
+        )
+
+        model = context.get("model") or ""
+        reasoning_config = context.get("reasoning_config")
+        base_url = context.get("base_url") or self.base_url
+
+        raw_thinking_config = _build_gemini_thinking_config(model, reasoning_config)
+        if not raw_thinking_config:
+            return {}
+
+        body: dict[str, Any] = {}
+        if self.name == "gemini" and _is_gemini_openai_compat_base_url(base_url):
+            thinking_config = _snake_case_gemini_thinking_config(raw_thinking_config)
+            if thinking_config:
+                body["extra_body"] = {"google": {"thinking_config": thinking_config}}
+        else:
+            body["thinking_config"] = raw_thinking_config
+        return body
+
+
+gemini = GeminiProfile(
+    name="gemini",
+    aliases=("google", "google-gemini", "google-ai-studio"),
+    api_mode="chat_completions",
+    env_vars=("GOOGLE_API_KEY", "GEMINI_API_KEY"),
+    base_url="https://generativelanguage.googleapis.com/v1beta",
+    auth_type="api_key",
+    default_aux_model="gemini-3-flash-preview",
+)
+
+google_gemini_cli = GeminiProfile(
+    name="google-gemini-cli",
+    aliases=("gemini-cli", "gemini-oauth"),
+    api_mode="chat_completions",
+    env_vars=(),  # OAuth — no API key
+    base_url="cloudcode-pa://google",  # Cloud Code Assist internal scheme
+    auth_type="oauth_external",
+)
+
+register_provider(gemini)
+register_provider(google_gemini_cli)