Add reproducible dev test target for optional MCP coverage

[hwesterb]

From PR #46 follow-up.

Evidence:

• pyproject.toml configures pytest testpaths for src/sema/mcp/tests, but there is no dev/test optional dependency group or documented one-command test setup.
• The local environment used for PR Align MCP validation with canonical schema #46 has neither pytest nor the optional mcp package installed.
• MCP tests now skip cleanly when mcp is absent, which keeps base installs from failing, but full MCP behavior still needs a dedicated environment with semahash[mcp].

Expected:

• Contributors and CI can run the same documented test targets.
• Base test runs should not require optional MCP dependencies.
• A separate MCP/full target should install the mcp extra and execute src/sema/mcp/tests without skipping.

Acceptance criteria:

• Add a documented command such as a dev extra, tox/nox target, or equivalent project-native workflow.
• CI runs at least a base test target and an MCP-extra test target.
• PR Align MCP validation with canonical schema #46's sema_validate coverage runs in the MCP-extra target rather than only being collected as a skip in base installs.