Add provider and request headers across SDKs

[stephentoub]

Expose the provider and request headers now available in the runtime.

[Copilot]

Pull request overview

This PR aligns the four SDKs (Node, Python, Go, .NET) on a minimal header-related surface area by exposing provider-level headers and per-message requestHeaders/request_headers, and ensuring these values are forwarded on the corresponding JSON-RPC calls (session.create, session.resume, session.send) with focused tests verifying the serialized request shapes.

Changes:

• Add provider headers to the public provider configuration surface and forward it through create/resume flows.
• Add per-turn requestHeaders/request_headers to message/send options and forward it on session.send.
• Add targeted serialization/request-shape tests in each SDK to validate the wire payloads.

Show a summary per file

File	Description
python/test_client.py	Adds pytest coverage asserting provider headers and per-turn request headers are forwarded in create/resume/send RPC payloads.
python/copilot/session.py	Adds request_headers to send/send_and_wait and forwards it as requestHeaders in the RPC params.
python/copilot/client.py	Extends provider snake_case→wire conversion to include headers.
nodejs/test/client.test.ts	Adds vitest coverage asserting provider headers and requestHeaders are present in create/resume/send requests.
nodejs/src/types.ts	Extends public TS types to include ProviderConfig.headers and MessageOptions.requestHeaders.
nodejs/src/session.ts	Forwards requestHeaders on the session.send request payload.
go/types_test.go	Adds JSON marshal/unmarshal tests ensuring headers and requestHeaders serialize under correct JSON property names.
go/types.go	Adds ProviderConfig.Headers and MessageOptions.RequestHeaders, and includes requestHeaders in the send request struct.
go/session.go	Wires MessageOptions.RequestHeaders into the session.send request.
dotnet/test/SerializationTests.cs	Adds serialization tests for ProviderConfig.Headers, MessageOptions.RequestHeaders, and the internal send request payload.
dotnet/src/Types.cs	Adds ProviderConfig.Headers and MessageOptions.RequestHeaders (with clone support for the dictionary).
dotnet/src/Session.cs	Forwards RequestHeaders into the internal SendMessageRequest and includes it in the record schema.

Copilot's findings

• Files reviewed: 12/12 changed files
• Comments generated: 0