Add opaque metadata passthrough to SDK tool definitions

[belaltaher8]

Add an optional _metadata`` bag to the Tooldefinition (anddefineTool`) and forward it verbatim over the session.create/resume RPC. This lets hosts attach namespaced, opaque metadata to tools without expanding the typed public contract; unknown keys are round-tripped untouched.

[Copilot]

Pull request overview

Adds an optional, opaque metadata bag to tool definitions across the multi-language SDKs and forwards it verbatim over session.create / session.resume, enabling hosts to attach namespaced, runtime-recognized (but SDK-opaque) attributes without expanding the typed contract.

Changes:

• Add metadata to tool definition types/builders (Rust/Go/Node/Python/.NET/Java) with “omit when unset” behavior where applicable.
• Wire metadata through session create/resume payload construction (Node + Python shown here; others rely on serialization of the definition type).
• Add/extend unit tests to validate serialization and omission semantics; update Java annotation-processor fixtures for the new constructor arity.

Show a summary per file

File	Description
rust/src/types.rs	Add Tool.metadata, builder, Debug output, and serialization test.
python/copilot/tools.py	Add Tool.metadata + define_tool(..., metadata=...) passthrough.
python/copilot/client.py	Include metadata when building tool definitions for create/resume RPCs.
python/test_client.py	Add test asserting metadata forwarded on create/resume and omitted when unset.
nodejs/src/types.ts	Extend Tool / defineTool types to include optional metadata.
nodejs/src/client.ts	Forward tool.metadata into session.create / session.resume payloads.
nodejs/test/client.test.ts	Add tests for forwarding and omission of metadata.
go/types.go	Add Tool.Metadata with omitempty JSON tag.
go/client_test.go	Add serialization tests for tool metadata presence/omission.
dotnet/src/CopilotTool.cs	Add MetadataKey + option plumbing into AITool.AdditionalProperties.
dotnet/src/Client.cs	Include metadata in internal wire ToolDefinition built from AI function declarations.
dotnet/test/Unit/CopilotToolTests.cs	Add unit tests ensuring metadata is set/omitted in additional properties.
java/src/main/java/com/github/copilot/rpc/ToolDefinition.java	Add metadata record component + factories + fluent modifier.
java/src/main/java/com/github/copilot/tool/CopilotToolProcessor.java	Emit null metadata argument in generated ToolDefinition(...) calls.
java/src/test/java/com/github/copilot/ToolDefinitionTest.java	Add serialization tests for metadata presence/omission.
java/src/test/java/com/github/copilot/rpc/fixtures/StaticTools$$CopilotToolMeta.java	Update fixture constructor call to include null metadata arg.
java/src/test/java/com/github/copilot/rpc/fixtures/StaticInvocationTools$$CopilotToolMeta.java	Update fixture constructor call to include null metadata arg.
java/src/test/java/com/github/copilot/rpc/fixtures/SimpleTools$$CopilotToolMeta.java	Update fixture constructor calls to include null metadata arg.
java/src/test/java/com/github/copilot/rpc/fixtures/OverrideTools$$CopilotToolMeta.java	Update fixture constructor call to include null metadata arg.
java/src/test/java/com/github/copilot/rpc/fixtures/OptionalParamTools$$CopilotToolMeta.java	Update fixture constructor calls to include null metadata arg.
java/src/test/java/com/github/copilot/rpc/fixtures/MultiReturnTools$$CopilotToolMeta.java	Update fixture constructor calls to include null metadata arg.
java/src/test/java/com/github/copilot/rpc/fixtures/InvocationAwareTools$$CopilotToolMeta.java	Update fixture constructor calls to include null metadata arg.
java/src/test/java/com/github/copilot/rpc/fixtures/DefaultValueTools$$CopilotToolMeta.java	Update fixture constructor call to include null metadata arg.
java/src/test/java/com/github/copilot/rpc/fixtures/DateTimeTools$$CopilotToolMeta.java	Update fixture constructor call to include null metadata arg.
java/src/test/java/com/github/copilot/rpc/fixtures/ArgCoercionTools$$CopilotToolMeta.java	Update fixture constructor call to include null metadata arg.
java/src/test/java/com/github/copilot/e2e/ErgonomicTestTools$$CopilotToolMeta.java	Update fixture constructor calls to include null metadata arg.

Review details

• Files reviewed: 26/26 changed files
• Comments generated: 1
• Review effort level: Low