fix(sdk): allow adapter-specific authentication

[gauravch-code]

Fixes #313.

The SDK currently recognizes only a fixed set of authentication methods, so applications cannot select newer adapter methods or configure custom gateway logins.

Adds optional authentication.selectMethod and authentication.clientCapabilities settings to connect and start. The selector receives the agent's advertised methods and can return a request with metadata, or skip authentication. Explicit authentication errors reach the caller and failed connections are closed. Legacy automatic selection stays unchanged when no selector is supplied.

Verification

• Three regression cases failed on the original SDK through the real Windows server; the legacy-authentication control passed.
• Seven authentication tests now pass against the published Sandbox Agent 0.5.3 Windows binary, using the repository's deterministic mock agent over real HTTP. No fetch or transport mocks.
• 19 provider lifecycle tests and two protocol-client smoke tests pass.
• SDK and test type checks, SDK bundle/declaration build, and changed-file formatting pass on Node 24.

Docker-backed SDK integration tests and live Claude/Codex gateway validation were not run.