Enable errorlint for error comparisons and type assertions (#7084)

## Changes
Enable `errorlint` with only its comparison and type-assertion checks,
and fix its 29 findings: `err == ErrX` becomes `errors.Is(err, ErrX)`
and `err.(*T)` becomes `errors.AsType[*T](err)`. The `errorf` check
(`%v` → `%w` in `fmt.Errorf`) stays off: wrapping makes the cause
visible to every `errors.Is`/`errors.As` up the stack (e.g.
`fs.ErrNotExist`, `context.Canceled`), so converting its 36 sites is a
behavior change that needs per-site review in a separate PR.

## Why
These checks break silently as soon as the error is wrapped with `%w`.
No current site is broken, since every sentinel is still returned
unwrapped, but the codebase was inconsistent: for example
`cmd/bundle/validate.go` compared `root.ErrAlreadyPrinted` with `!=`
while `cmd/bundle/utils/process.go` uses `errors.Is`.

## Tests
golangci-lint reports 0 issues on the root and tools modules. Unit tests
pass for every changed package, and the `cmd/` and `bundle/validate/`
acceptance tests pass. One test keeps its `!=` with a
`//nolint:errorlint`, because it checks that `mapUploadError` returns
the same error rather than a wrapper.

This pull request and its description were written by Isaac.

---------

Co-authored-by: Isaac <no-reply@databricks.com>

Author: denik

.golangci.yaml

@@ -35,7 +35,13 @@ linters:
     - reassign
     - modernize
     - makezero
+    - errorlint
   settings:
+    errorlint:
+      # Switching %v to %w changes which errors callers can match with
+      # errors.Is/As (e.g. fs.ErrNotExist, context.Canceled), so it needs
+      # per-site review rather than a bulk fix.
+      errorf: false
     modernize:
       # omitzero changes JSON wire format for nested structs (unlike omitempty),
       # so each site needs per-site review rather than a bulk rewrite.

acceptance/acceptance_test.go

@@ -1648,7 +1648,7 @@ func formatOutput(w io.Writer, err error) {
 	if err == nil {
 		return
 	}
-	if exiterr, ok := err.(*exec.ExitError); ok {
+	if exiterr, ok := errors.AsType[*exec.ExitError](err); ok {
 		exitCode := exiterr.ExitCode()
 		fmt.Fprintf(w, "\nExit code: %d\n", exitCode)
 	} else {
@@ -1859,7 +1859,7 @@ func runWithLog(t *testing.T, cmd *exec.Cmd, out *os.File, tail bool, timeout ti
 				require.NoError(t, err)
 			}
 		}
-		if err == io.EOF {
+		if errors.Is(err, io.EOF) {
 			break
 		}
 		require.NoError(t, err)

acceptance/internal/prepare_server.go

@@ -3,6 +3,7 @@ package internal
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"mime"
@@ -376,7 +377,7 @@ func normalizeMultipartBody(req *testserver.Request) (any, bool) {
 	parts := map[string]any{}
 	for {
 		part, err := mr.NextPart()
-		if err == io.EOF {
+		if errors.Is(err, io.EOF) {
 			break
 		}
 		if err != nil {

bundle/artifacts/tarball_test.go

@@ -4,6 +4,7 @@ import (
 	"archive/tar"
 	"bytes"
 	"compress/gzip"
+	"errors"
 	"io"
 	"os"
 	"os/exec"
@@ -26,7 +27,7 @@ func tarEntries(t *testing.T, b []byte) map[string]string {
 	out := map[string]string{}
 	for {
 		hdr, err := tr.Next()
-		if err == io.EOF {
+		if errors.Is(err, io.EOF) {
 			break
 		}
 		require.NoError(t, err)

bundle/config/mutator/python/python_mutator.go

@@ -293,7 +293,7 @@ func (m *pythonMutator) Apply(ctx context.Context, b *bundle.Bundle) diag.Diagno
 	b.Metrics.PythonUpdatedResourcesCount += int64(result.UpdatedResources.Size())
 	b.Metrics.PythonAddedResourcesCount += int64(result.AddedResources.Size())
 
-	if err == mutateDiagsHasError {
+	if errors.Is(err, mutateDiagsHasError) {
 		if !mutateDiags.HasError() {
 			panic("mutateDiags has no error, but error is expected")
 		}

cmd/air/logstream_test.go

@@ -62,10 +62,10 @@ func TestClassifyLogError(t *testing.T) {
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			got := classifyLogError(tt.err)
-			switch tt.want {
-			case errBricklensFeatureDisabled:
+			switch {
+			case errors.Is(tt.want, errBricklensFeatureDisabled):
 				assert.ErrorIs(t, got, errBricklensFeatureDisabled)
-			case nil:
+			case tt.want == nil:
 				assert.NoError(t, got)
 			default:
 				assert.ErrorIs(t, got, tt.want)

cmd/auth/env.go

@@ -79,7 +79,7 @@ func loadFromDatabricksCfg(ctx context.Context, cfg *config.Config) error {
 		return err
 	}
 	profile, err := resolveSection(cfg, iniFile)
-	if err == ErrNoMatchingProfiles {
+	if errors.Is(err, ErrNoMatchingProfiles) {
 		// it's also fine for Azure CLI or Databricks CLI, which
 		// are resolved by unified auth handling in the Go SDK.
 		return nil

cmd/bundle/validate.go

@@ -2,6 +2,7 @@ package bundle
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 
 	"github.com/databricks/cli/bundle"
@@ -67,7 +68,7 @@ Please run this command before deploying to ensure configuration quality.`,
 		})
 		ctx := cmd.Context()
 
-		if err != nil && err != root.ErrAlreadyPrinted {
+		if err != nil && !errors.Is(err, root.ErrAlreadyPrinted) {
 			logdiag.LogError(ctx, err)
 			err = root.ErrAlreadyPrinted
 		}

experimental/ssh/internal/client/agentshim.go

@@ -378,7 +378,7 @@ func probeModelServices(ctx context.Context, apiClient *client.DatabricksClient)
 	it := catalog.NewAiGateway(apiClient).ListModelServices(ctx, catalog.ListModelServicesRequest{PageSize: modelServiceProbePageSize})
 	_, err := it.Next(ctx)
 	if err != nil {
-		if err == listing.ErrNoMoreItems {
+		if errors.Is(err, listing.ErrNoMoreItems) {
 			// endpoint is reachable but there are no results
 			return gatewayProbe{reachable: true}
 		}

libs/appproxy/appproxy.go

@@ -2,6 +2,7 @@ package appproxy
 
 import (
 	"context"
+	"errors"
 	"io"
 	"net"
 	"net/http"
@@ -113,7 +114,7 @@ func (p *Proxy) handleWebSocket(w http.ResponseWriter, r *http.Request) {
 	err = <-errc
 	if err != nil {
 		// If the error is not EOF, then there was a problem
-		if err != io.EOF {
+		if !errors.Is(err, io.EOF) {
 			// Log the error and perform cleanup
 			log.Warnf(r.Context(), "error copying messages: %v", err)
 			middlewareConn.Close()