feat(mcp): migrate to mcp 2.x (MCPServer), replacing the <2 ceiling

Pilot for moving the fleet off mcp 1.x before it goes away. The port is
three mechanical changes plus docs:

  - import:      mcp.server.fastmcp.FastMCP -> mcp.server.mcpserver.MCPServer
  - constructor: transport options moved off it; MCPServer(name) only
  - run():       host/port/stateless_http/transport_security are now kwargs,
                 and `mcp.settings` no longer exists

`@mcp.tool()` is unchanged — all six tools keep their decorators and
signatures untouched.

Verified, not assumed:

1. tools/list is byte-for-byte identical between mcp 1.27.1 and 2.x
   (6 tools, same names, same inputSchema properties and required lists,
   dumped with by_alias=True so the comparison is in wire format). This
   was the real risk: 2.x switched the protocol types to snake_case
   internally, and tool schemas are what decide routing.
2. The server boots for real over streamable-http with stateless_http=True
   and DNS-rebinding protection disabled, and answers initialize 200 with
   the same capability set.
3. CI now runs `import docs_mcp.server` inside the built image before push.

One behavior change: an unversioned server reports serverInfo.version ""
under 2.x instead of the SDK version, so Drawbar will see an empty version
string. Cosmetic — left as-is rather than inventing a version scheme.

Dependency note: 2.x has no [fastmcp] extra, replaces httpx with httpx2,
and requires opentelemetry-api. Our own httpx>=0.27 pin is unaffected —
httpx2 is a separate distribution.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01FnVuG79cYPcRLTp4pC8ujR

README.md

@@ -147,7 +147,7 @@ PRODUCT_NAME=crop_seed python -m docs_mcp.server \
 │   ├── index.py                   # Chroma + BM25 builder
 │   └── bm25.py                    # FTS5 lexical index w/ seed-domain facets
 ├── docs_mcp/
-│   ├── server.py                  # FastMCP — 6 tools, hybrid+rerank
+│   ├── server.py                  # MCPServer — 6 tools, hybrid+rerank
 │   ├── lessons.md                 # Curated knowledge layer (Pioneer fallback)
 │   └── usage.py                   # TimedCall + JSONL telemetry
 ├── eval/