## Stack Context
PR 3 of 6 in a stack that migrates every Coder MCP surface from the
archived `github.com/mark3labs/mcp-go` library to the official
`github.com/modelcontextprotocol/go-sdk` v1.7.0.
Stack: #28056 -> #28057 -> #28058 -> #28059 -> #28060 -> #28061
## Why
The chatd external MCP client (admin-configured MCP servers used by
Agent chat) now holds `*mcp.ClientSession` connections created via
`mcp.NewClient` and `Client.Connect`, with `StreamableClientTransport`
or `SSEClientTransport` per server config.
- Auth and identity headers are injected through a custom
`http.RoundTripper` because the official SDK has no per-header transport
options.
- Tool input schemas are extracted from the SDK's `map[string]any`
decoding.
- Content conversion handles the official pointer content types; the SDK
decodes blob resources into raw bytes, so binary content is handled
without an extra base64 round trip.
- Test fixtures are official stateless Streamable HTTP servers.
> Mux created this PR on Mike's behalf.
In our codebase we have an existing convention of using
`flag.Lookup("test.v")` instead of `testing.Testing()`. This avoids
pulling in the entire `testing` package. Another consequence: some of
our custom linters trigger upon import of the `testing` package which
can lead to unexpected linter errors.
Use testing.Testing() inside createTransport to automatically
clone http.DefaultTransport when running in tests. In production,
DefaultTransport is used as-is (efficient connection pooling).
This fixes the CloseIdleConnections flake class: httptest.Server.Close()
calls http.DefaultTransport.CloseIdleConnections(), which disrupts
any MCP client sharing that transport. The testing.Testing() check
means every MCP transport created during tests gets isolation
automatically, with no caller changes needed.
Closescoder/internal#1016
Closes PLAT-291
Michael Suchacz