feat(mcp): migrate to mcp 2.x (MCPServer), lift the <2 pin

mcp 2.0.0 removed `mcp.server.fastmcp`. This repo was pinned to <2
(6723829) to stop a rebuild from crash-looping the container the way
seed-mcp (2026-09-01) and zerto-docs (2026-08-11) did. This lifts the
pin by doing the port — the same mechanical change as
justin/seed-mcp#23 and justin/morpheus-docs#10.

- import: FastMCP -> MCPServer, plus TransportSecuritySettings
- constructor takes no transport options: MCPServer(f"{PRODUCT_NAME}-docs")
- host/port/stateless_http/transport_security are run() kwargs;
  `mcp.settings` no longer exists
- requirement becomes plain `mcp>=2,<3` (2.x has no [fastmcp] extra)

@mcp.tool() decorators and all handler signatures are unchanged.

CI now smokes `import docs_mcp.server` twice in both workflows — once
after pip install, once inside the built image — so a green build can
never ship a non-importing container again.

Verified on python:3.12-slim (matches the image):
- tools/list dumped in wire format is BYTE-FOR-BYTE IDENTICAL between
  mcp 1.30.0 and 2.2.0, all 10 tools
- streamable-http boots; initialize returns HTTP 200, both from
  localhost and with a container-DNS `Host: hvm-docs-mcp:8000` header
  (passing host= to run() keeps DNS-rebinding protection off)
- no mcp-session-id response header, i.e. stateless_http is in effect
- stdio boots; initialize + tools/list return 10 tools
- production image builds and imports under mcp 2.2.0; httpx 0.28.1 and
  httpx2 2.12.0 coexist as expected

Retrieval is untouched, so no eval numbers are included.

Closes #12

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01QBx2P1VhKQEzJWoZ96QcnH

requirements.txt

@@ -1,11 +1,6 @@
 # MCP server
-# Pinned below 2.0: mcp 2.0.0 removed `mcp.server.fastmcp`, which this
-# server imports. The unpinned floor pulled 2.0.0 into a rebuild on
-# 2026-08-11 and crash-looped zerto-docs with
-# "ModuleNotFoundError: No module named 'mcp.server.fastmcp'".
-# 2.0 also drops the [fastmcp] extra. Migrating to the 2.x API is a
-# deliberate piece of work — do not lift this pin without it.
-mcp[fastmcp]>=1.0.0,<2
+# 2.x: no [fastmcp] extra; FastMCP -> MCPServer (see docs_mcp/server.py).
+mcp>=2,<3
 pydantic>=2.0
 httpx>=0.27