fix(packaging): define MCP optional dependency

[alooshxl]

Closes #608.

Problem

When MCP is missing, create_server() points users at an extra that the package does not define:

# agent_reach/integrations/mcp_server.py
"MCP not installed. Install: python -m pip install "
"'agent-reach[mcp] @ "
"https://github.com/Panniantong/agent-reach/archive/main.zip'",

[project.optional-dependencies] on main declares browser, cookies, all, and dev — there is no mcp extra. Following the printed command therefore reinstalls Agent Reach without the MCP dependency, and the next run prints the same message:

$ python -m pip install --dry-run ".[mcp]"     # on main
WARNING: agent-reach 1.5.0 does not provide the extra 'mcp'
Would install agent-reach-1.5.0

$ python -m pip install --dry-run ".[mcp]"     # on this branch
Would install ... mcp-2.0.0 mcp-types-2.0.0 ... typer-0.27.1 uvicorn-0.52.3 ...

Change

• pyproject.toml: add mcp = ["mcp[cli]>=1.0"], reusing the exact requirement already listed in the all extra.
• tests/test_mcp_server.py: add two regressions — one asserting the [project.optional-dependencies] block declares that extra, one asserting that with HAS_MCP false create_server() exits with status 1 and names agent-reach[mcp] on stderr.

The metadata test reads pyproject.toml as UTF-8 text instead of parsing TOML, following the existing precedent in tests/test_youtube_channel.py, so it needs no TOML parser on Python 3.10. The missing-dependency test patches HAS_MCP and does not require MCP to be uninstalled from the test environment.

Verification

Run on this branch (Windows, Python 3.14.5, pytest 9.1.1, ruff 0.16.3, mypy 2.3.0):

Command	Result
python -m pytest tests/test_mcp_server.py -v	4 passed
python -m ruff check agent_reach tests	All checks passed
python -m mypy agent_reach	Success: no issues found in 35 source files
git diff --check	clean
python -m pytest tests/ -v	4 failed, 556 passed, 28 skipped, 16 subtests passed
python -m pip install --dry-run ".[mcp]"	resolves mcp-2.0.0 (warns about an unknown extra on main)

The metadata test was confirmed to fail before the pyproject.toml edit and to pass after it. MCP is not installed in the environment these tests ran in, so the missing-dependency path is exercised for real.

The 4 failures are pre-existing in this local environment and unrelated to this change: the symlink security tests in tests/test_channels.py and tests/test_reddit_channel.py raise OSError: [WinError 1314] A required privilege is not held by the client because this Windows account cannot create symlinks. They fail identically on main before this change (baseline: 4 failed, 554 passed, 28 skipped), and #616 addresses them separately.

Scope

• No change to agent_reach/integrations/mcp_server.py, its tool contract, or its install message.
• MCP stays optional: base dependencies are untouched and the all extra is unchanged.
• No version bump, changelog entry, constraints change, or documentation edit.