MCP polish: dedupe wrappers, fill test gaps

[robinbowes]

Goal

Small tech-debt cleanup in the MCP layer that built up while shipping
Phase 4 quickly.

Sub-tasks

• Collapse _wrap / _wrap_mutation triplication. The same
  helper pair lives in mcp/tools/lifecycle.py,
  mcp/tools/fields.py, and mcp/tools/relations.py.
  Extract to a single shared module under mcp/tools/_wrap.py (or
  mcp/_helpers.py).
• Add a test for _require_mcp_sdk error path. Currently no
  coverage for the case where the mcp package is missing —
  important because that path is what users hit when they install
  without the [mcp] extra.
• Lift coverage in tests/mcp/test_tools_relations.py to
  match its sibling test files. Relations cover tags / contacts /
  links — three sub-surfaces, currently thinner test cases than
  fields/lifecycle.

Design considerations

• Keep the wrapper signature compatible — call sites won't change.
• The missing-SDK test should patch importlib.import_module (or
  the sentinel that _require_mcp_sdk checks), not actually
  uninstall the package.

Out of scope

• Restructuring the tool registration mechanism.
• Public API changes.

Cost estimate

Half-day total.

[yo61-lastlight]

Triage Summary

Category: enhancement
Summary: Clean up three tech-debt items in the MCP layer: deduplicate the _wrap/_wrap_mutation helpers, add a test for the missing-SDK error path, and improve relations tool test coverage.

Current behavior:
The _wrap/_wrap_mutation helper pair is duplicated verbatim in three MCP tool modules. There is no test coverage for the _require_mcp_sdk error path (the failure mode users hit when the [mcp] extra is not installed). The relations tool tests are thinner than the field/lifecycle sibling test files.

Desired behavior:
The _wrap/_wrap_mutation helpers live in a single shared module; all three tool modules import from it. The _require_mcp_sdk error path has at least one test covering the missing-package case (patched at the import level, not by uninstalling the package). Relations tool tests cover tags, contacts, and links at coverage parity with the field and lifecycle test files.

Acceptance criteria:

• _wrap and _wrap_mutation extracted to a single shared location; no duplication across tool modules
• Existing call sites unchanged — no API breakage
• A test exercises the _require_mcp_sdk failure path using a mock/patch rather than package removal
• tests/mcp/test_tools_relations.py coverage for tags, contacts, and links matches the depth of the lifecycle and fields test files

Out of scope:

• Restructuring the tool registration mechanism
• Any public API changes