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).

run_agent.py

@@ -7871,6 +7871,7 @@ class AIAgent:
             or base_url_host_matches(self.base_url, "moonshot.ai")
             or base_url_host_matches(self.base_url, "moonshot.cn")
         )
+        _is_tokenhub = base_url_host_matches(self._base_url_lower, "tokenhub.tencentmaas.com")
 
         # Temperature: _fixed_temperature_for_model may return OMIT_TEMPERATURE
         # sentinel (temperature omitted entirely), a numeric override, or None.
@@ -7942,6 +7943,7 @@ class AIAgent:
             is_github_models=_is_gh,
             is_nvidia_nim=_is_nvidia,
             is_kimi=_is_kimi,
+            is_tokenhub=_is_tokenhub,
             is_custom_provider=self.provider == "custom",
             ollama_num_ctx=self._ollama_num_ctx,
             provider_preferences=_prefs or None,
@@ -7989,6 +7991,7 @@ class AIAgent:
             "x-ai/",
             "google/gemini-2",
             "qwen/qwen3",
+            "tencent/hy3-preview",
         )
         return any(model.startswith(prefix) for prefix in reasoning_model_prefixes)