Add provider runtime metadata: supportsServer and isAvailable()

[sroussey]

Summary

Introduces runtime metadata and availability probing for AI providers to support deployment-aware provider selection and credential threading. Adds supportsServer property to all providers and implements isAvailable() probe for environment-dependent providers like Chrome Built-in AI.

Key Changes

• New abstract property supportsServer on AiProvider base class to declare whether a provider can run server-side (Bun/Node), orthogonal to isLocal. Defaults to true in cloud provider mixin.

• New isAvailable() method on AiProvider for runtime environment probing. Returns true by default; overridden by environment-dependent providers (e.g., Chrome AI checks for window.LanguageModel, window.Summarizer, etc.).

• Updated CloudProviderMetadata interface to include optional supportsServer field, enabling declarative metadata for cloud providers (Anthropic, OpenAI, Ollama).

• Implemented isAvailable() override in WebBrowserProvider to check for Chrome AI globals, replacing the renderer's former ad-hoc isChromeBuiltinAiAvailable call with a uniform provider-level probe.

• Updated all provider implementations (Cactus, HuggingFace Transformers, MLX, llama.cpp, TensorFlow MediaPipe, and test providers) to declare supportsServer property.

• Added comprehensive test coverage:

  • ProviderRuntimeMetadata.test.ts: Validates metadata declarations across all providers, verifies renderer-only providers cannot be cloud-reachable, and ensures credential threading safety.
  • ProviderBaseUrlSeam.test.ts: Contract tests for HTTP provider base_url repointing (Decision S-07b).

Implementation Details

• supportsServer is orthogonal to isLocal: a provider can be local (Ollama) yet server-capable, or cloud-based and not local.
• The isAvailable() probe is async to support future environment checks that may require async operations.
• Chrome Built-in AI checks five API globals (LanguageModel, Summarizer, Translator, Rewriter, LanguageDetector) to determine availability.
• Test notes document known discrepancies: Anthropic and OpenAI currently default supportsBrowser=true via mixin, though spec calls for false (deferred per task report).

https://claude.ai/code/session_018FeLmEJ4iJWrnGMAg8idUK

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	62.11%	24617 / 39631
🔵	Statements	61.97%	25471 / 41100
🔵	Functions	63.04%	4662 / 7395
🔵	Branches	50.79%	12071 / 23763

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
providers/huggingface-transformers/src/ai/HuggingFaceTransformersProvider.ts	0%	100%	0%	0%	28-64
providers/huggingface-transformers/src/ai/HuggingFaceTransformersQueuedProvider.ts	7.14%	0%	0%	7.14%	29-120

Generated in workflow #2444 for commit 678f125 by the Vitest Coverage Report Action