Add built-in plugin directory support

[lutzroeder]

Summary

• add host-bundled built-in plugin directory startup options across the SDKs
• register the complete built-in plugin path set with plugins.builtin.set after connect/protocol verification and before sessions can be created
• document the distinction between ordinary --plugin-dir loading and trusted host-bundled plugin registration

Closes #2327

Validation

• cargo test --features test-support --test builtin_plugin_directories_test

[Copilot]

Pull request overview

Adds trusted, host-bundled plugin directory registration across all supported SDKs.

Changes:

• Adds absolute-path startup options for six SDKs.
• Registers directories through plugins.builtin.set after protocol verification.
• Adds tests and documentation for the new behavior.

Show a summary per file

File	Description
rust/src/lib.rs	Adds Rust option and startup RPC.
rust/tests/builtin_plugin_directories_test.rs	Tests Rust registration and validation.
python/copilot/client.py	Adds Python option and startup RPC.
python/test_client.py	Tests Python behavior.
nodejs/src/types.ts	Defines the TypeScript option.
nodejs/src/client.ts	Validates paths and registers plugins.
nodejs/test/client.test.ts	Tests Node.js behavior.
java/src/main/java/com/github/copilot/rpc/CopilotClientOptions.java	Adds Java configuration API.
java/src/main/java/com/github/copilot/CopilotClient.java	Registers paths during Java startup.
java/src/test/java/com/github/copilot/BuiltinPluginDirectoriesTest.java	Tests Java behavior.
go/types.go	Defines the Go option.
go/client.go	Validates and registers Go paths.
go/client_test.go	Tests Go behavior.
dotnet/src/Types.cs	Defines and clones the .NET option.
dotnet/src/Client.cs	Validates and registers .NET paths.
dotnet/test/Unit/CloneTests.cs	Tests option cloning.
dotnet/test/Unit/GitHubTelemetryTests.cs	Tests .NET startup registration.
docs/features/plugin-directories.md	Documents trusted plugin directories.

Review details

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• Files reviewed: 18/18 changed files
• Comments generated: 2
• Review effort level: Balanced

[github-actions]

Cross-SDK Consistency Review ✅

This PR adds builtinPluginDirectories / builtin_plugin_directories across all six SDKs (Node.js, Python, Go, .NET, Java, Rust). The implementation is consistent across languages:

Aspect	Status
Option naming	✅ Follows each language's convention (builtinPluginDirectories, builtin_plugin_directories, BuiltinPluginDirectories)
Absolute path validation	✅ All SDKs validate at construction/init time
Validation error type	✅ Each SDK uses its idiomatic error (panic in Go's NewClient matches existing patterns there; exceptions/errors elsewhere)
RPC call	✅ All SDKs call plugins.builtin.set with {"paths": [...]}
Timing	✅ All SDKs register before sessions can be created
Failure handling	✅ All SDKs clean up on RPC failure
Tests	✅ Both Java and Rust have new dedicated test files

No consistency gaps found. The PR maintains full feature parity across all SDK implementations.

Generated by SDK Consistency Review Agent for #2330 · sonnet46 32 AIC · ⌖ 5.43 AIC · ⊞ 6.6K · ◷