feat: add coder_workspace_read_file MCP tool (#19562)

Follows similarly to the bash tool (and some code to connect to an agent
was extracted from it).

There are two main parts: a new agent endpoint, and then a new MCP tool
that consumes that endpoint.

codersdk/workspacesdk/agentconn.go

@@ -60,6 +60,7 @@ type AgentConn interface {
 	PrometheusMetrics(ctx context.Context) ([]byte, error)
 	ReconnectingPTY(ctx context.Context, id uuid.UUID, height uint16, width uint16, command string, initOpts ...AgentReconnectingPTYInitOption) (net.Conn, error)
 	RecreateDevcontainer(ctx context.Context, devcontainerID string) (codersdk.Response, error)
+	ReadFile(ctx context.Context, path string, offset, limit int64) (io.ReadCloser, string, error)
 	SSH(ctx context.Context) (*gonet.TCPConn, error)
 	SSHClient(ctx context.Context) (*ssh.Client, error)
 	SSHClientOnPort(ctx context.Context, port uint16) (*ssh.Client, error)
@@ -476,6 +477,30 @@ func (c *agentConn) RecreateDevcontainer(ctx context.Context, devcontainerID str
 	return m, nil
 }
 
+// ReadFile reads from a file from the workspace, returning a file reader and
+// the mime type.
+func (c *agentConn) ReadFile(ctx context.Context, path string, offset, limit int64) (io.ReadCloser, string, error) {
+	ctx, span := tracing.StartSpan(ctx)
+	defer span.End()
+
+	//nolint:bodyclose // we want to return the body so the caller can stream.
+	res, err := c.apiRequest(ctx, http.MethodGet, fmt.Sprintf("/api/v0/read-file?path=%s&offset=%d&limit=%d", path, offset, limit), nil)
+	if err != nil {
+		return nil, "", xerrors.Errorf("do request: %w", err)
+	}
+	if res.StatusCode != http.StatusOK {
+		// codersdk.ReadBodyAsError will close the body.
+		return nil, "", codersdk.ReadBodyAsError(res)
+	}
+
+	mimeType := res.Header.Get("Content-Type")
+	if mimeType == "" {
+		mimeType = "application/octet-stream"
+	}
+
+	return res.Body, mimeType, nil
+}
+
 // apiRequest makes a request to the workspace agent's HTTP API server.
 func (c *agentConn) apiRequest(ctx context.Context, method, path string, body io.Reader) (*http.Response, error) {
 	ctx, span := tracing.StartSpan(ctx)

codersdk/workspacesdk/agentconnmock/agentconnmock.go

@@ -232,6 +232,22 @@ func (mr *MockAgentConnMockRecorder) PrometheusMetrics(ctx any) *gomock.Call {
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "PrometheusMetrics", reflect.TypeOf((*MockAgentConn)(nil).PrometheusMetrics), ctx)
 }
 
+// ReadFile mocks base method.
+func (m *MockAgentConn) ReadFile(ctx context.Context, path string, offset, limit int64) (io.ReadCloser, string, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "ReadFile", ctx, path, offset, limit)
+	ret0, _ := ret[0].(io.ReadCloser)
+	ret1, _ := ret[1].(string)
+	ret2, _ := ret[2].(error)
+	return ret0, ret1, ret2
+}
+
+// ReadFile indicates an expected call of ReadFile.
+func (mr *MockAgentConnMockRecorder) ReadFile(ctx, path, offset, limit any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "ReadFile", reflect.TypeOf((*MockAgentConn)(nil).ReadFile), ctx, path, offset, limit)
+}
+
 // ReconnectingPTY mocks base method.
 func (m *MockAgentConn) ReconnectingPTY(ctx context.Context, id uuid.UUID, height, width uint16, command string, initOpts ...workspacesdk.AgentReconnectingPTYInitOption) (net.Conn, error) {
 	m.ctrl.T.Helper()