Lazy load MCP SDK client

[Jason Winters (jwin321)]

Cached Learn MCP sessions can complete direct tool calls without initializing the SDK client or HTTP transport. This change defers those imports and client construction until the SDK fallback is actually needed, reducing startup work on the common cache fast path.

A unit test now confirms the cached direct-call path does not construct the SDK client.

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[Copilot]

Pull request overview

Defers MCP SDK client and transport initialization until the SDK fallback path is needed, improving cached direct-call startup performance.

Changes:

• Lazily loads and caches the SDK client and transport.
• Adds a regression test confirming cached calls skip SDK construction.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
cli/test/unit/cache.test.ts	Verifies the cached direct-call path avoids SDK construction.
cli/src/mcp/client.ts	Implements lazy SDK client and transport initialization.

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