build: cap mcp dependency to <2 (fixes #235)

[DUC750]

Problem

mcp-proxy declares an unbounded mcp>=1.27.1. mcp 2.0.0 removed request_ctx from mcp.server.lowlevel.server, which src/mcp_proxy/proxy_server.py imports at module load. As a result, any fresh resolution (uvx mcp-proxy, a clean pip install, or a Docker rebuild) pulls mcp==2.0.0 and crashes on startup:

ImportError: cannot import name 'request_ctx' from 'mcp.server.lowlevel.server'

This matches the report in #235.

Fix

Cap the dependency at mcp>=1.27.1,<2 so resolution stays on the 1.x line that still provides request_ctx. This is the minimal change that restores a working install; it does not add support for mcp 2.x (that would require migrating proxy_server.py off the removed request_ctx API and can be a follow-up). The cap can be lifted/raised once the code targets the 2.x request-context API.

Verified locally (alpine 3.24, Python 3.14, mcp-proxy 0.12.0):

• without the cap → resolves mcp==2.0.0 → import mcp_proxy.proxy_server raises the ImportError above
• with the cap → resolves mcp==1.29.0 → import succeeds

Closes #235.

[RenanMsV]

This provides a simple and straightforward way to address the issue until MCP 2.0.0 support is available.