fix(mcp): make the [mcp] extra installable and the server constructible

[phaestos2501]

Problem

Three issues prevent the MCP server from starting from a fresh pip install .[mcp]:

1. fastmcp is undeclared. fle/env/protocols/_mcp/__init__.py and fle/mcp_dataloader.py import it, but the mcp extra only lists mcp[cli] and dulwich, so the package falls into the (broken, see 3) fallback path.
2. Unpinned mcp resolves to 2.0, which removed McpError from mcp.shared.exceptions and breaks fastmcp at import (fastmcp 2.14 only requires mcp>=1.24 with no upper bound).
3. Server construction fails and the fallback crashes. Current fastmcp releases removed the dependencies constructor kwarg, so FastMCP(..., dependencies=[...]) raises TypeError — which the except ImportError guard doesn't catch. And when fastmcp is genuinely missing, the module still crashes on mcp.lifespan = fle_lifespan with mcp = None. Additionally, fastmcp only honours a lifespan passed to the constructor (stored in _lifespan at init), so the post-hoc attribute assignment was silently ignored even when it didn't crash — sessions would run without the Factorio lifecycle manager.

Fix

• Declare fastmcp>=2.14.0 and pin mcp[cli]>=1.24.0,<2 in the mcp extra.
• Define fle_lifespan before the server is created and pass it as the lifespan constructor kwarg (its late-bound module globals resolve by the time a session starts). The no-fastmcp fallback now imports cleanly with mcp = None.

Verification

• uv pip install -e '.[mcp]' resolves and installs (previously missing fastmcp entirely).
• FastMCP('probe', lifespan=fle_lifespan) constructs and _lifespan is wired.
• Fallback path verified by blocking the fastmcp import: module imports cleanly with mcp = None.
• Full end-to-end (tools served against a running cluster) not yet exercised: importing the package eagerly builds FactorioMCPState at import time, which requires live Factorio containers — arguably its own issue. A fresh install additionally hits the a2a-sdk 1.x API break addressed by Fixes related to dependencies #372.

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