feat(model/openai): add Responses API ChatModel

[jesse-engineer]

What type of PR is this?

feat

Check the PR title.

• This PR title matches the conventional commit format.
• The description is user-oriented and clear.
• Separate user-documentation PR (English/Chinese README documentation and a runnable example are included here).

Detailed description

Applications using schema.Message and ToolCallingChatModel currently need a custom adapter to call /v1/responses. This adds NewResponsesChatModel and ResponsesChatModelConfig to components/model/openai, so these applications can use Responses with their existing message history and middleware. NewChatModel continues to use Chat Completions.

The new adapter supports generation and streaming, text/image inputs, custom function calls and tool choice, Eino callbacks, response/request IDs, token usage, and optional stored-response continuation. Encrypted reasoning items are retained in Message.Extra and restored after session serialization. Failed/cancelled responses and truncated streams return errors; refusals and output limits retain meaningful finish reasons. SDK retries are disabled for application/ADK retry ownership.

An optional StreamEventHandler lets applications classify gateway-specific SSE events using their own error types. Immediate errors stop and close the stream; incomplete-stream errors apply only when no terminal response arrives. Standard terminal responses and SDK/transport errors retain precedence, and fallback state is isolated per stream. No gateway-specific event handling is built into the component.

agenticopenai remains the provider-native AgenticMessage implementation. This proposal addresses the classic-message compatibility use case discussed in #758 and #782; it does not expose hosted tools or arbitrary native content blocks.

Compatibility: the official SDK dependency is github.com/openai/openai-go/v3 v3.35.0, matching the version currently used by agenticopenai on main. This raises this module's minimum Go version from 1.18 to 1.22. The Eino dependency remains v0.7.13.

Validation

• Go 1.25.6: component tests, new example compilation, and go test -race -gcflags="all=-N -l" . pass.
• Go 1.22.12: component tests and example compilation pass (external linking used for the local macOS linker).
• 25 Responses tests cover request options, tool continuation, encrypted-reasoning JSON round trips, streaming, usage, errors, refusals, and response-ID continuation.
• Component statement coverage: 75.5%.
• golangci-lint (govet, ineffassign, unused): 0 issues; goimports and git whitespace checks pass.
• Stream-event tests cover error identity, immediate connection closure, successful responses with quota notices, native error precedence, and concurrent stream isolation.
• A downstream integration check using the consumer’s real ADK retry/failover policies passed 9 scenarios, including retry exhaustion followed by successful failover for both quota and upstream errors.
• Tests use local HTTP fixtures and require no API credentials.