feat(provider): make openai, anthropic, google, and amazon-bedrock optional

[dgageot]

Projects embedding docker-agent can now drop model providers they don't need, shrinking both the binary and transitive dependency graph. The largest win is dropping the AWS SDK (aws-sdk-go-v2) via the docker_agent_no_bedrock tag, which eliminates the entire Bedrock integration and its 61-package dependency tree.

The monolithic pkg/model/provider/factory.go was split into a shared dispatch core plus one self-registering file per optional provider, each gated by a build tag. Providers register into the dispatch map via init(), and a model whose provider was compiled out fails at construction time with a clear "not compiled into this build" error. All providers are included by default; pass a tag to opt out of unwanted ones.

Build flags (all included by default; use tags to exclude):

• docker_agent_no_openai — drops github.com/openai/openai-go
• docker_agent_no_anthropic — drops github.com/anthropics/anthropic-sdk-go
• docker_agent_no_google — drops google.golang.org/genai and Vertex AI / cloud auth stack
• docker_agent_no_bedrock — drops the github.com/aws/aws-sdk-go-v2 stack

Example: go build -tags 'docker_agent_no_bedrock docker_agent_no_openai' ...

Tags are prefixed with docker_agent_ because Go build tags are global to a build (not scoped per module), so a bare no_openai would collide with an embedding project's own tags. The rationale and full tag list are documented in the package doc via go doc / pkg.go.dev.

One caveat: anthropic and openai SDKs are also imported by the google provider's Vertex Model Garden support, so they only fully drop when combined with docker_agent_no_google. The openai-go and genai packages also appear elsewhere (dmr/oaistream, chatserver, pkg/a2a), so those won't leave the module graph entirely. The dmr provider and rule-based router remain always compiled in. Builds pass with every tag combination and without any tags; js/wasm still builds; lint is clean.

[docker-agent]

Assessment: 🟢 APPROVE

The provider opt-out refactoring is well-structured. The split from a monolithic factory.go into per-provider self-registering files (factory_openai.go, factory_anthropic.go, factory_google.go, factory_bedrock.go) is clean and correct. Build tag syntax (//go:build !docker_agent_no_<provider>) is syntactically valid. The providerFactories map is initialized before init() runs, and the registerProviderFactory panic-on-duplicate acts as an intentional safety net against misconfigured builds. All four providers are included by default; the docker_agent_ prefix on tags avoids collision with embedding projects.

No high or medium severity bugs were found in the changed code.

Minor hygiene notes (no action required):

• A few test files (custom_provider_test.go, etc.) lack build-tag guards matching their factory dependencies. This is harmless today since none of those tests call New() with a provider-specific path, but adding guards would make the intent explicit.
• schema_test.go always imports SDK sub-packages regardless of build tags, so test binaries won't fully tree-shake even when provider tags are set — this is expected given those sub-packages appear elsewhere in the module.