[DOCS] Document mcts serve [api] extra requirement in doctor output

[hello-args]

Summary

mcts serve requires the optional [api] install extra (FastAPI + uvicorn). Base pip install mcp-mcts users get exit 2 with a brief error — but mcts doctor never mentions the [api] extra, so the REST API feature appears broken out of the box. Documented in getting-started; not surfaced at preflight time.

Status: BY DESIGN — optional extra is intentional; gap is discoverability, not missing functionality.

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Doc path

TBD — file path or URL

Problem

MCTS packaging splits heavy dependencies:

Command	Extra	Dependencies
mcts scan	core	minimal
mcts scan --live	[mcp]	MCP SDK
mcts serve	[api]	FastAPI, uvicorn
mcts-mcp	[mcp]	MCP SDK

Getting-started documents this:

pip install "mcp-mcts[api]" # for mcts serve

Users trying mcts serve after core-only install hit ImportError path.

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Expected

Doctor should mention uv sync --extra api.

Acceptance Criteria

• Doctor warns when [api] extra missing
• Doctor pass line when [api] installed
• serve error references doctor check
• getting-started unchanged (already documents extra)
• No false fail on core-only installs (warn only)

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