chore: retire mark3labs/mcp-go dependency - #28061

## Stack Context

PR 2 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

`coder exp mcp server` (stdio) now uses the official SDK server with
`mcp.IOTransport` over the invocation's stdin/stdout, and reuses the
shared `coderd/mcp.RegisterSDKTool` helper from PR #28056 so both
servers register tools identically.

- A `nopWriteCloser` prevents the SDK from closing the invocation's
stdout.
- Tests send spec-compliant initialize params and
`notifications/initialized` before `tools/list` because the official SDK
enforces the protocol lifecycle.

> Mux created this PR on Mike's behalf.

…al Go SDK (#28058)

## 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.

## Stack Context

PR 1 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, adding MCP 2026-07-28
support while keeping compatibility with clients speaking 2024-11-05
through 2025-06-18.

Stack: coder#28056 -> coder#28057 -> coder#28058 -> coder#28059 -> coder#28060 -> coder#28061

## Why

The coderd Streamable HTTP MCP server (`/api/experimental/mcp/http`) is
the foundation layer: it introduces the official SDK dependency and the
shared `RegisterSDKTool` helper the CLI server reuses.

- The server runs the SDK handler in stateless mode with `JSONResponse:
true`, preserving the previous `application/json` POST wire format. GET
and DELETE return 405, and no `Mcp-Session-Id` is issued, both permitted
by the Streamable HTTP spec.
- `DisableLocalhostProtection` is set because coderd commonly listens on
loopback behind a reverse proxy with a public Host header; the
endpoint's bearer authentication is the relevant access control.
- Tool registration builds raw JSON object schemas and omits empty
`required`, keeping `tools/list` output byte-identical to the previous
server (verified with a golden comparison).
- SDK logs are adapted to `cdr.dev/slog/v3`; only warnings and errors
are forwarded because the SDK logs several INFO lines per stateless
request.
- Tests cover the modern 2026-07-28 flow, legacy 2025-06-18 initialize,
unsupported protocol version rejection (`-32022`), and non-POST method
behavior.

## Known behavior deltas vs the old endpoint

Both deltas come from the SDK enforcing the Streamable HTTP spec where
mark3labs was lenient, on an experimental endpoint:

- POST requests whose `Accept` header lists `application/json` without
`text/event-stream` are now rejected with 400 (the spec requires clients
to list both; a missing `Accept` header is still tolerated). mark3labs
did not validate `Accept` at all.
- The old server generated an unvalidated `Mcp-Session-Id` response
header; the stateless SDK handler issues none. Clients that merely echo
the header back are unaffected.

## Validation

Beyond unit/integration tests, a remote dogfood UAT ran protocol
conformance against a live dev server built from the stack tip: version
negotiation matrix (2024-11-05 through bogus/omitted values), auth,
session/method semantics, tool schema sanity, tools/call happy and error
paths (unknown tool, schema-violating args, malformed JSON, jsonrpc
"1.0"), and a concurrency smoke test. No 500s or connection drops; error
shapes are clean JSON-RPC/HTTP errors.

> Mux created this PR on Mike's behalf.

…l Go SDK (#28059)

## Stack Context

PR 4 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 workspace agent MCP manager now stores `*mcp.ClientSession` per
configured server.

- stdio servers use `mcp.CommandTransport` with an `exec.Cmd` built from
Coder's `agentexec.Execer`, preserving environment enrichment; the
command uses the manager's parent context so a stdio subprocess outlives
the connect handshake and stops when the session closes.
- HTTP and SSE servers use header-injecting HTTP clients.
- Binary tool content is re-encoded to base64 for the agent API because
the official SDK decodes it into raw bytes.
- The reload test now triggers config diffs via an environment variable
because the official SDK drops connections on non-protocol stdout output
(flags like `-test.v` made the fake server chatty).

> Mux created this PR on Mike's behalf.