fix: return null from getShellFromEnv when environment variables are not set

- Modified getShellFromEnv() to return null instead of hardcoded fallback values when COMSPEC (Windows) or SHELL (Unix) environment variables are not set
- This ensures the function accurately reflects whether the shell path was obtained from environment variables
- Updated test comments to clarify the expected behavior with the fix
- All existing tests continue to pass as the fallback logic is handled by getSafeFallbackShell()

Fixes #8578

Author: roomote

src/utils/__tests__/shell.spec.ts

@@ -247,6 +247,8 @@ describe("Shell Detection Tests", () => {
 
 		it("falls back to /bin/zsh if no config, userInfo, or env variable is set", () => {
 			vscode.workspace.getConfiguration = () => ({ get: () => undefined }) as any
+			// With the fix, getShellFromEnv() returns null when SHELL is not set,
+			// so it falls back to the safe default from getSafeFallbackShell()
 			expect(getShell()).toBe("/bin/zsh")
 		})
 	})
@@ -303,6 +305,8 @@ describe("Shell Detection Tests", () => {
 
 		it("falls back to /bin/bash if nothing is set", () => {
 			vscode.workspace.getConfiguration = () => ({ get: () => undefined }) as any
+			// With the fix, getShellFromEnv() returns null when SHELL is not set,
+			// so it falls back to the safe default from getSafeFallbackShell()
 			expect(getShell()).toBe("/bin/bash")
 		})
 	})
@@ -345,6 +349,8 @@ describe("Shell Detection Tests", () => {
 				throw new Error("userInfo error")
 			})
 			delete process.env.SHELL
+			// With the fix, getShellFromEnv() returns null when SHELL is not set,
+			// so it falls back to the safe default from getSafeFallbackShell()
 			expect(getShell()).toBe("/bin/bash")
 		})
 	})

src/utils/shell.ts

@@ -269,18 +269,14 @@ function getShellFromEnv(): string | null {
 
 	if (process.platform === "win32") {
 		// On Windows, COMSPEC typically holds cmd.exe
-		return env.COMSPEC || "C:\\Windows\\System32\\cmd.exe"
+		return env.COMSPEC || null
 	}
 
-	if (process.platform === "darwin") {
-		// On macOS/Linux, SHELL is commonly the environment variable
-		return env.SHELL || "/bin/zsh"
+	// On Unix-like systems (macOS, Linux), SHELL is the environment variable
+	if (process.platform === "darwin" || process.platform === "linux") {
+		return env.SHELL || null
 	}
 
-	if (process.platform === "linux") {
-		// On Linux, SHELL is commonly the environment variable
-		return env.SHELL || "/bin/bash"
-	}
 	return null
 }