Remove SkipEngineEnvVar from ProcessOptions (#4759)

## Changes

- Remove `SkipEngineEnvVar` field from `ProcessOptions` in `process.go`
- In `migrate.go`, clear `DATABRICKS_BUNDLE_ENGINE` in the context via
`env.Set(ctx, engine.EnvVar, "")` before calling `ProcessBundleRet`,
achieving the same effect

## Why
Simplify env var handling for
#4749

## Test plan
Existing tests.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: denik

cmd/bundle/deployment/migrate.go

@@ -9,13 +9,15 @@ import (
 	"os/exec"
 	"strings"
 
+	"github.com/databricks/cli/bundle/config/engine"
 	"github.com/databricks/cli/bundle/deploy/terraform"
 	"github.com/databricks/cli/bundle/deployplan"
 	"github.com/databricks/cli/bundle/direct"
 	"github.com/databricks/cli/bundle/direct/dstate"
 	"github.com/databricks/cli/cmd/bundle/utils"
 	"github.com/databricks/cli/cmd/root"
 	"github.com/databricks/cli/libs/cmdio"
+	"github.com/databricks/cli/libs/env"
 	"github.com/databricks/cli/libs/logdiag"
 	"github.com/databricks/cli/libs/shellquote"
 	"github.com/databricks/cli/libs/structs/structaccess"
@@ -140,9 +142,12 @@ WARNING: Both direct deployment engine and this command are experimental and not
 	cmd.RunE = func(cmd *cobra.Command, args []string) error {
 		extraArgs, extraArgsStr := getCommonArgs(cmd)
 
+		// Clear the engine env var so migrate always uses terraform engine to read existing state,
+		// regardless of what the user may have set in their environment.
+		cmd.SetContext(env.Set(cmd.Context(), engine.EnvVar, ""))
+
 		opts := utils.ProcessOptions{
-			SkipEngineEnvVar: true,
-			AlwaysPull:       true,
+			AlwaysPull: true,
 			// Same options as regular deploy, to ensure bundle config is in the same state
 			FastValidate: true,
 			Build:        true,

cmd/bundle/utils/process.go

@@ -59,9 +59,6 @@ type ProcessOptions struct {
 	// If true, configure outputHandler for phases.Deploy
 	Verbose bool
 
-	// If true, do not read DATABRICKS_BUNDLE_ENGINE env var (for migrate command, which ignores this env var)
-	SkipEngineEnvVar bool
-
 	// If true, call corresponding phase:
 	FastValidate    bool
 	Validate        bool
@@ -94,13 +91,9 @@ func ProcessBundleRet(cmd *cobra.Command, opts ProcessOptions) (*bundle.Bundle,
 		cmd.SetContext(ctx)
 	}
 
-	requiredEngine := engine.EngineNotSet
-
-	if !opts.SkipEngineEnvVar {
-		requiredEngine, err = engine.FromEnv(ctx)
-		if err != nil {
-			return nil, nil, err
-		}
+	requiredEngine, err := engine.FromEnv(ctx)
+	if err != nil {
+		return nil, nil, err
 	}
 
 	// Load bundle config and apply target