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).

tests/hermes_cli/test_xiaomi_provider.py

@@ -84,7 +84,8 @@ class TestXiaomiAutoDetection:
                      "DASHSCOPE_API_KEY", "XAI_API_KEY", "KIMI_API_KEY",
                      "MINIMAX_API_KEY", "AI_GATEWAY_API_KEY", "KILOCODE_API_KEY",
                      "HF_TOKEN", "GLM_API_KEY", "COPILOT_GITHUB_TOKEN",
-                     "GH_TOKEN", "GITHUB_TOKEN", "MINIMAX_CN_API_KEY"):
+                     "GH_TOKEN", "GITHUB_TOKEN", "MINIMAX_CN_API_KEY",
+                     "TOKENHUB_API_KEY", "ARCEEAI_API_KEY"):
             monkeypatch.delenv(var, raising=False)
         monkeypatch.setenv("XIAOMI_API_KEY", "sk-xiaomi-test-12345678")
         provider = resolve_provider("auto")