Pin mcp SDK to patched fork for stateless task leak fix (#631)

## Why

The upstream MCP Python SDK has a memory leak where stateless HTTP
request tasks accumulate in the global task group when clients
disconnect mid-request
([modelcontextprotocol/python-sdk#1764](modelcontextprotocol/python-sdk#1764)).
This causes unbounded memory growth in long-running MCP servers like our
remote dbt MCP proxy.

The fix (request-scoped task groups) also introduces a regression where
SSE streams don't close cleanly on shutdown — reverse proxies see
connection resets instead of clean HTTP close. A graceful drain step
resolves this.

## What

Points the `mcp` dependency at
`dbt-labs/mcp-python-sdk@dbt-labs/patched`, which includes:
- Request-scoped task groups for stateless requests (prevents zombie
task accumulation)
- Graceful SSE drain on shutdown (terminates all active transports
before cancelling the task group)

Upstream PR:
[modelcontextprotocol/python-sdk#2145](modelcontextprotocol/python-sdk#2145)

## Notes

This is a temporary pin until the upstream PR is merged and released.
Once upstream publishes a release containing the fix, we should revert
to the published package.

Drafted by Claude Opus 4.6 under the direction of @wiggzz

Author: wiggzz

.changes/unreleased/Under the Hood-20260310-mcp-patched-sdk.yaml

@@ -0,0 +1,3 @@
+kind: Under the Hood
+body: Pin mcp SDK to dbt-labs/patched fork for stateless task leak fix
+time: 2026-03-10T14:00:00.000000-07:00

pyproject.toml

@@ -58,6 +58,9 @@ dev = [
   "types-authlib>=1.6.4.20250920",
 ]
 
+[tool.uv.sources]
+mcp = { git = "https://github.com/dbt-labs/mcp-python-sdk.git", branch = "dbt-labs/patched" }
+
 [project.urls]
 Documentation = "https://docs.getdbt.com/docs/dbt-ai/about-mcp"
 Issues = "https://github.com/dbt-labs/dbt-mcp/issues"