[WIP] ExternalPush scaler doesn't respect other scalers activity to scale to zero

[Copilot]

Fix ExternalPush Scaler Scale-to-Zero Issue ✅

Problem: ExternalPush scalers currently don't respect other scalers' activity when scaling to zero. When an external push scaler reports inactive, it immediately scales to zero even if other scalers are still active (e.g., HTTP + cron scenario).

Solution Plan:

• Analyze current implementation in startPushScalers and GetPushScalers
• Understand the relationship with scaling modifiers
• Implement fix in startPushScalers to check all scalers before scaling to zero
• Update GetPushScalers to exclude push scalers when scaling modifiers are used
• Add proper error handling and logging
• Test the changes
• Verify the fix handles the reported use case (HTTP + cron scenario)

Key Changes:

1. Modified startPushScalers: When an external push scaler reports inactive, now checks activity of all other scalers before scaling to zero
2. Updated GetPushScalers: Returns empty array when IsUsingModifiers() is true, as suggested in the issue
3. Added comprehensive tests: Created tests to validate the scaling modifiers behavior

Implementation Details:

• Only external push scalers reporting inactive trigger the additional scaler check
• Active external push scalers still scale up immediately (preserving fast scale-from-zero)
• When scaling modifiers are used, push scalers are disabled entirely
• Maintains thread safety with existing mutex patterns

Validation Results:
✅ Scenario 1: HTTP push inactive + Cron active → Does NOT scale to zero
✅ Scenario 2: All scalers inactive → DOES scale to zero
✅ Scenario 3: With scaling modifiers → Push scalers disabled
✅ Scenario 4: Without scaling modifiers → Push scalers enabled
✅ All existing tests pass: External push scaler tests, cache tests, etc.

Files Modified:

• pkg/scaling/cache/scalers_cache.go - Updated GetPushScalers() method
• pkg/scaling/scale_handler.go - Enhanced startPushScalers() logic
• pkg/scaling/cache/scalers_cache_test.go - Added comprehensive tests

Fixes #6986

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