Refactor interpretTaskModes for better readability and performance

[Copilot]

The interpretTaskModes function was hard to read with complex conditional logic and performed redundant iterations by calling someTasksAreOnly multiple times on the same suites.

Changes

Performance

• Cache someTasksAreOnly results in a Map per suite traversal instead of recomputing recursively
• Use Set for testLocations lookup (O(1) vs O(n) array search)

Readability

• Merge duplicate skipAllTasks/todoAllTasks into single setModeForAllTasks(suite, mode)
• Restructure conditionals into clear sequential sections with comments
• Use nullish coalescing (??, ??=) for defaults
• Add default parameters to inner traverseSuite function

Testing

• Add 29 unit tests covering: basic modes, .only handling, name pattern filtering, location filtering, allowOnly errors, and nested suite scenarios

// Before: someTasksAreOnly called multiple times per task
const hasSomeTasksOnly = onlyMode && suite.tasks.some(
  t => t.mode === 'only' || (t.type === 'suite' && someTasksAreOnly(t)),
)
const includeTask = hasSomeTasksOnly
  ? (t.mode === 'only' || (t.type === 'suite' && someTasksAreOnly(t)))  // redundant call
  : ...

// After: pre-compute once per suite
let hasOnlyChild: Map<Suite, boolean> | undefined
if (onlyMode) {
  hasOnlyChild = new Map()
  for (const task of suite.tasks) {
    if (task.type === 'suite') {
      hasOnlyChild.set(task, someTasksAreOnly(task))
    }
  }
}

Original prompt

interpretTaskModes seems to be very complicated.

1. it's hard to read
2. it does a lot of extra iterations
   Rewrite it to be more performant and readable. Create integration tests first and validate the tests pass, and then start refactoring.

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