feat(models): add OrcaRouter as a named model provider - #6688

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Link to Issue or Description of Change

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Problem:
Model names routed through an OpenAI-compatible aggregator that ADK does not yet know about fall through to LiteLLM; if LiteLLM does not list the provider, resolution fails with Model not found.

Solution:
Adds OrcaRouter as a model provider. OrcaRouter is an OpenAI-compatible model routing gateway that exposes 150+ models from OpenAI, Anthropic, Google, DeepSeek, Qwen, MiniMax, xAI and others behind a single endpoint and API key. It also provides gateway-level security controls for AI agents.

This PR registers a named OrcaRouterLlm provider that mirrors the existing Claude subclass of AnthropicLlm: it only overrides client construction so requests go to OrcaRouter's Anthropic-compatible /v1/messages endpoint at https://api.orcarouter.ai, reading the key from ORCAROUTER_API_KEY. Model names like orcarouter/anthropic/claude-haiku-4.5 now resolve directly to the named provider, and orcarouter:... prefix syntax is supported as well.

Testing Plan

Please describe the tests that you ran to verify your changes. This is required
for all PRs that are not small documentation or typo fixes.

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Added tests/unittests/models/test_orca_router_llm.py (supported models, default model, model-name prefix stripping, client construction with ORCAROUTER_API_KEY / base URL, and the missing-credential error) and a registry resolution test in tests/unittests/models/test_models.py.

$ python -m pytest tests/unittests/models/ -q
1022 passed, 36 warnings in 33.36s

Manual End-to-End (E2E) Tests:

Live smoke test against the real OrcaRouter endpoint (ORCAROUTER_API_KEY set to a placeholder) through OrcaRouterLlm.generate_content_async reached https://api.orcarouter.ai/v1/messages and returned an OrcaRouter-level 401 AuthenticationError (orcarouter_api_error), confirming the base URL and request path are wired correctly.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

Contributions to this project require a signed Google Contributor License Agreement; this change is submitted on that basis.

I'm an engineer on the OrcaRouter team.