chore(deps): bump mcp to 1.28.1 (security)

Clears 20 high-severity Dependabot alerts for the MCP Python SDK across the
root lock and five integration locks (integration-tests, claude-agent-sdk,
crewai, openai-agents, strands):

  GHSA-jpw9-pfvf-9f58  HTTP transports serve session requests without
                       verifying the authenticated principal   (patched 1.27.2)
  GHSA-hvrp-rf83-w775  experimental task handlers let any client access/
                       cancel other clients' tasks              (patched 1.27.2)
  GHSA-vj7q-gjh5-988w  WebSocket server transport lacks Host/Origin
                       validation                               (patched 1.28.1)

1.28.1 clears all three. mcp is a direct dep in hindsight-integration-tests
and claude-agent-sdk (mcp>=1.0.0) and transitive elsewhere; the locks just
pinned older versions (1.23.3–1.27.1). crewai jumped the furthest (1.23.3),
which pulled newer pydantic/pydantic-core graph edges — its tests still pass.

Not included here: mcp is not part of any Dependabot group PR, so this is
the sole coverage for these alerts. nltk (llamaindex/pipecat) and torch are
handled by the Dependabot uv-group PR #2780.

Verified: claude-agent-sdk 76 passed, crewai 35 passed; lint clean.

Author: dcbouius