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/vercel.py

@@ -0,0 +1,43 @@
+"""Vercel AI Gateway provider profile.
+
+AI Gateway routes to multiple backends. Hermes sends attribution
+headers and full reasoning config passthrough.
+"""
+
+from typing import Any
+
+from providers import register_provider
+from providers.base import ProviderProfile
+
+
+class VercelAIGatewayProfile(ProviderProfile):
+    """Vercel AI Gateway — attribution headers + reasoning passthrough."""
+
+    def build_api_kwargs_extras(
+        self,
+        *,
+        reasoning_config: dict | None = None,
+        supports_reasoning: bool = True,
+        **ctx: Any,
+    ) -> tuple[dict[str, Any], dict[str, Any]]:
+        extra_body: dict[str, Any] = {}
+        if supports_reasoning and reasoning_config is not None:
+            extra_body["reasoning"] = dict(reasoning_config)
+        elif supports_reasoning:
+            extra_body["reasoning"] = {"enabled": True, "effort": "medium"}
+        return extra_body, {}
+
+
+vercel = VercelAIGatewayProfile(
+    name="ai-gateway",
+    aliases=("vercel", "vercel-ai-gateway", "ai_gateway", "aigateway"),
+    env_vars=("AI_GATEWAY_API_KEY",),
+    base_url="https://ai-gateway.vercel.sh/v1",
+    default_headers={
+        "HTTP-Referer": "https://hermes-agent.nousresearch.com",
+        "X-Title": "Hermes Agent",
+    },
+    default_aux_model="google/gemini-3-flash",
+)
+
+register_provider(vercel)