Add defer parameter to tool definition

[almaleksia]

This PR adds defer parameter to tool definition that allows to configure tool deferral and pre-loading when tool search is activated.

[Copilot]

Pull request overview

This PR adds an optional defer parameter to tool definitions across the SDKs, enabling the runtime/CLI to lazily load tools via tool search (or force pre-loading) based on an explicit deferral mode.

Changes:

• Introduces a defer/Defer field (auto/never) on tool definitions across Rust, Python, Node.js, Go, Java, and .NET.
• Wires defer through session create/resume request payloads where tools are serialized.
• Adds/updates unit tests and READMEs to document and validate defer serialization/omission behavior.

Show a summary per file

File	Description
rust/src/types.rs	Adds defer: Option<DeferMode> to Tool, plus enum + builder + serialization test.
python/test_client.py	Adds tests asserting defer is sent on session.create/session.resume.
python/README.md	Documents Python tool deferral usage via defer="auto" | "never".
python/copilot/tools.py	Adds defer to Tool and define_tool(...) API + docstring.
python/copilot/client.py	Forwards tool.defer into tool definitions for create/resume payloads.
nodejs/test/client.test.ts	Adds tests asserting defer is included in tool defs for create/resume.
nodejs/src/types.ts	Extends Tool/defineTool types with optional defer?: "auto" | "never".
nodejs/src/client.ts	Forwards defer into tool definitions for create/resume payloads.
nodejs/README.md	Documents Node tool deferral usage via defer: "auto" | "never".
java/src/test/java/com/github/copilot/ToolDefinitionTest.java	Adds serialization tests for Java ToolDefinition.defer.
java/src/main/java/com/github/copilot/rpc/ToolDefinition.java	Adds defer field to RPC tool definition + new factory method.
java/src/main/java/com/github/copilot/rpc/ToolDefer.java	Introduces ToolDefer enum with JSON (de)serialization.
go/types.go	Adds ToolDefer type + Tool.Defer JSON field.
go/README.md	Documents Go tool deferral usage (ToolDeferAuto / ToolDeferNever).
go/client_test.go	Adds tests for defer serialization and omission when unset.
dotnet/test/Unit/CopilotToolTests.cs	Extends unit tests to cover Defer metadata propagation.
dotnet/src/CopilotTool.cs	Adds CopilotToolDefer + CopilotToolOptions.Defer and metadata wiring.
dotnet/src/Client.cs	Adds Defer to internal RPC tool definition mapping + source-gen metadata.
dotnet/README.md	Documents .NET tool deferral usage (CopilotToolOptions.Defer).

Copilot's findings

• Files reviewed: 19/19 changed files
• Comments generated: 1

[SteveSandersonMS]

The naming confuses me a bit. Is CopilotToolDefer.Auto exactly equivalent to not setting a value, or does it mean something different?

[almaleksia]

Yes it is the same - value defaults to "auto" currently. Fixed all occurences in docs/comments.

[SteveSandersonMS]

Great, thanks!