fix: server-side diffs and stricter fuzzy splicing for edit_files (#24454)

Fixes three classes of edit_files bugs and adds structured per-file
diff output for tool callers:

- New IncludeDiff flag on FileEditRequest; when set, the agent
  returns FileEditResponse.Files[]{Path, Diff} with unified diffs
  computed via go-udiff v0.4.1 Lines + ToUnified (not Unified,
  which calls log.Fatalf on internal error).
- Fuzzy match comparators split each line into leading whitespace,
  body, trailing whitespace, and ending. The splice substitutes at
  each position: on agreement between search and replace the file's
  bytes win; on disagreement the replacement's bytes are spliced
  verbatim. Carve-outs for empty-body lines, multi-line EOF splices,
  and level-aware indent translation for inserted lines.
- Indent-unit detection (GCD for spaces, tab-priority) lets a 4sp
  LLM search insert correctly into tab or 2sp files. Falls back to
  the previous cLead-inheritance path when units can't be detected
  cleanly.
- Empty search is rejected with "search string must not be empty".
- Duplicate file paths in one request are rejected; symlink aliases
  resolved via api.resolvePath before the dedup check.
- Frontend EditFilesRenderer consumes the structured files array by
  explicit path (no label munging) with per-file synthetic fallback
  for older agents or mismatched paths. On error, no diff is
  rendered so the synthetic fallback doesn't misrepresent a
  rejected edit as applied.

Breaking change: AgentConn.EditFiles changes from (ctx, req) error
to (ctx, req) (FileEditResponse, error) in codersdk/workspacesdk.
Source-breaking for external Go consumers; no compat shim per plan
owner.

Out of scope (tracked in CODAGT-214): level-aware indent for
middle-substituted splice lines. Locked in
TestEditFiles_FuzzyIndent_InsertionLevelAware's Lock_* cases plus
TestEditFiles_ReplaceAll_FuzzyIndentGap.

codersdk/toolsdk/toolsdk.go

@@ -1665,7 +1665,19 @@ type WorkspaceEditFileArgs struct {
 	Edits     []workspacesdk.FileEdit `json:"edits"`
 }
 
-var WorkspaceEditFile = Tool[WorkspaceEditFileArgs, codersdk.Response]{
+// WorkspaceEditFilesResponse is the response shape for the edit-file
+// and edit-files tools. Message preserves the existing success text.
+// Files carries the per-file results returned by the agent
+// (populated when the agent-side IncludeDiff flag was set). The
+// field is named Files (matching the agent's FileEditResponse.Files)
+// so future per-file error or status fields can be added without a
+// second wire break.
+type WorkspaceEditFilesResponse struct {
+	Message string                        `json:"message"`
+	Files   []workspacesdk.FileEditResult `json:"files,omitempty"`
+}
+
+var WorkspaceEditFile = Tool[WorkspaceEditFileArgs, WorkspaceEditFilesResponse]{
 	Tool: aisdk.Tool{
 		Name:        ToolNameWorkspaceEditFile,
 		Description: `Edit a file in a workspace.`,
@@ -1703,27 +1715,29 @@ var WorkspaceEditFile = Tool[WorkspaceEditFileArgs, codersdk.Response]{
 	},
 	MCPAnnotations:     mcpDestructiveAnnotations,
 	UserClientOptional: true,
-	Handler: func(ctx context.Context, deps Deps, args WorkspaceEditFileArgs) (codersdk.Response, error) {
+	Handler: func(ctx context.Context, deps Deps, args WorkspaceEditFileArgs) (WorkspaceEditFilesResponse, error) {
 		conn, err := newAgentConn(ctx, deps.coderClient, args.Workspace)
 		if err != nil {
-			return codersdk.Response{}, err
+			return WorkspaceEditFilesResponse{}, err
 		}
 		defer conn.Close()
 
-		err = conn.EditFiles(ctx, workspacesdk.FileEditRequest{
+		resp, err := conn.EditFiles(ctx, workspacesdk.FileEditRequest{
 			Files: []workspacesdk.FileEdits{
 				{
 					Path:  args.Path,
 					Edits: args.Edits,
 				},
 			},
+			IncludeDiff: true,
 		})
 		if err != nil {
-			return codersdk.Response{}, err
+			return WorkspaceEditFilesResponse{}, err
 		}
 
-		return codersdk.Response{
+		return WorkspaceEditFilesResponse{
 			Message: "File edited successfully.",
+			Files:   resp.Files,
 		}, nil
 	},
 }
@@ -1733,7 +1747,7 @@ type WorkspaceEditFilesArgs struct {
 	Files     []workspacesdk.FileEdits `json:"files"`
 }
 
-var WorkspaceEditFiles = Tool[WorkspaceEditFilesArgs, codersdk.Response]{
+var WorkspaceEditFiles = Tool[WorkspaceEditFilesArgs, WorkspaceEditFilesResponse]{
 	Tool: aisdk.Tool{
 		Name:        ToolNameWorkspaceEditFiles,
 		Description: `Edit one or more files in a workspace.`,
@@ -1785,20 +1799,24 @@ var WorkspaceEditFiles = Tool[WorkspaceEditFilesArgs, codersdk.Response]{
 	},
 	MCPAnnotations:     mcpDestructiveAnnotations,
 	UserClientOptional: true,
-	Handler: func(ctx context.Context, deps Deps, args WorkspaceEditFilesArgs) (codersdk.Response, error) {
+	Handler: func(ctx context.Context, deps Deps, args WorkspaceEditFilesArgs) (WorkspaceEditFilesResponse, error) {
 		conn, err := newAgentConn(ctx, deps.coderClient, args.Workspace)
 		if err != nil {
-			return codersdk.Response{}, err
+			return WorkspaceEditFilesResponse{}, err
 		}
 		defer conn.Close()
 
-		err = conn.EditFiles(ctx, workspacesdk.FileEditRequest{Files: args.Files})
+		resp, err := conn.EditFiles(ctx, workspacesdk.FileEditRequest{
+			Files:       args.Files,
+			IncludeDiff: true,
+		})
 		if err != nil {
-			return codersdk.Response{}, err
+			return WorkspaceEditFilesResponse{}, err
 		}
 
-		return codersdk.Response{
+		return WorkspaceEditFilesResponse{
 			Message: "File(s) edited successfully.",
+			Files:   resp.Files,
 		}, nil
 	},
 }