feat(providers): add tencent-tokenhub provider support

Registers tencent-tokenhub (https://tokenhub.tencentmaas.com/v1) as a
new API-key provider with model tencent/hy3-preview (256K context).

- PROVIDER_REGISTRY entry + TOKENHUB_API_KEY / TOKENHUB_BASE_URL env vars
- Aliases: tencent, tokenhub, tencent-cloud, tencentmaas
- openai_chat transport with is_tokenhub branch for top-level
  reasoning_effort (Hy3 is a reasoning model)
- tencent/hy3-preview:free added to OpenRouter curated list
- 60+ tests (provider registry, aliases, runtime resolution,
  credentials, model catalog, URL mapping, context length)
- Docs: integrations/providers.md, environment-variables.md,
  model-catalog.json

Author: simonweng <simonweng@tencent.com>
Salvaged from PR #16860 onto current main (resolved conflicts with
#16935 Azure Anthropic env-var hint tests and the --provider choices=
list removal in chat_parser).

agent/transports/chat_completions.py

@@ -188,6 +188,7 @@ class ChatCompletionsTransport(ProviderTransport):
         anthropic_max_out = params.get("anthropic_max_output")
         is_nvidia_nim = params.get("is_nvidia_nim", False)
         is_kimi = params.get("is_kimi", False)
+        is_tokenhub = params.get("is_tokenhub", False)
         reasoning_config = params.get("reasoning_config")
 
         if ephemeral is not None and max_tokens_fn:
@@ -219,6 +220,21 @@ class ChatCompletionsTransport(ProviderTransport):
                         _kimi_effort = _e
                 api_kwargs["reasoning_effort"] = _kimi_effort
 
+        # Tencent TokenHub: top-level reasoning_effort (unless thinking disabled)
+        if is_tokenhub:
+            _tokenhub_thinking_off = bool(
+                reasoning_config
+                and isinstance(reasoning_config, dict)
+                and reasoning_config.get("enabled") is False
+            )
+            if not _tokenhub_thinking_off:
+                _tokenhub_effort = "high"
+                if reasoning_config and isinstance(reasoning_config, dict):
+                    _e = (reasoning_config.get("effort") or "").strip().lower()
+                    if _e in ("low", "medium", "high"):
+                        _tokenhub_effort = _e
+                api_kwargs["reasoning_effort"] = _tokenhub_effort
+
         # extra_body assembly
         extra_body: Dict[str, Any] = {}