typing: graduate opencontractserver.mcp.* from mypy baseline

[JSv4]

Sub-issue of #1447. Continues draining the mypy baseline established in #1331.

Scope

Three `opencontractserver.mcp.*` modules remain on the baseline:

• `opencontractserver.mcp.server`
• `opencontractserver.mcp.tools`
• `opencontractserver.mcp.tests.test_mcp`

`test_mcp` is the single largest baseline file at 319 errors (per `docs/typing/README.md`'s baseline-shape table). Recommend graduating `server` and `tools` first as one PR, then tackling `test_mcp` in a dedicated follow-up.

Acceptance

• Per-module or per-package PR(s)
• Modules' `[mypy-…]` blocks removed
• Pre-commit `mypy` runs clean on the full project surface

Notes

• `mcp.server` only serves public corpora — keep that contract intact.
• `test_mcp` is the worst-offender file (319 errors). Expect heavy use of MagicMock-driven tests with the same `MagicMock` auto-attribute pattern that hid Notification tests use MagicMock to hide bug in analyzer_name field path #1471 — graduating it is a chance to audit for similar real bugs.