test: add toBeVisible assertions before every click in E2E tests

Adds `await expect(locator).toBeVisible()` before `.click()` calls
across 104 test and fixture files (431 assertions). Gives immediate,
descriptive failures instead of generic actionability timeouts.

Skips force clicks, canvas/mouse coordinate clicks, custom click
methods, catch chains, and toPass retry blocks. Updates
FLAKE_PREVENTION_RULES.md with the new rule.

browser_tests/fixtures/helpers/NodeOperationsHelper.ts

@@ -1,4 +1,5 @@
 import type { Locator } from '@playwright/test'
+import { expect } from '@playwright/test'
 
 import type {
   LGraph,
@@ -179,6 +180,7 @@ export class NodeOperationsHelper {
       position: DefaultGraphPositions.emptyLatentWidgetClick
     })
     const dialogInput = this.page.locator('.graphdialog input[type="text"]')
+    await expect(dialogInput).toBeVisible()
     await dialogInput.click()
     await dialogInput.fill('128')
     await dialogInput.press('Enter')