Renamed TestGetVpcsAndSubnets to TestSubnetDiscovery and updated test expectations
to match the new targeted subnet discovery behavior. The optimization only
discovers subnets that are actually used by node instances, not all subnets
subscription-wide.

This aligns with the same test fix applied in PR cilium#41554 for v1.16 branch.

…xcept Azure SDK v2

This change ensures perfect alignment between PR cilium#41555 (main branch) and PR cilium#41554 (v1.16 branch):

- Fixed GetNodesSubnets signature to return only SubnetMap (not VNetMap + SubnetMap)
- Updated extractSubnetIDs to use memory-efficient map[string]struct{} instead of map[string]bool
- Aligned error handling in resyncInstances to continue with empty subnets on GetNodesSubnets failure
- Added missing TestExtractSubnetIDs test for deduplication validation
- Maintained three-phase subnet discovery optimization identical to PR cilium#41554

The only differences between the PRs are now Azure SDK version upgrades (v1 → v2).

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Final alignment between PR cilium#41555 (main branch) and PR cilium#41554 (v1.16 branch):

ALIGNED FEATURES:
- Pre-compiled regex pattern with expectedCaptureGroups constant
- getSubnetWithPagination method for accurate IP configuration counting
- Enhanced parseSubnetID function with proper error handling
- Subscription ID tracking in Client struct
- Proper logrus-based logging in GetNodesSubnets
- Three-phase subnet discovery optimization identical to PR cilium#41554

SDK V2 ADAPTATIONS:
- Uses armnetwork.SubnetsClient instead of network.SubnetsClient
- Simplified pagination logic using SDK v2 built-in capabilities
- Updated ARM response handling for result.Subnet access
- Removed SDK v1-specific Azure API version detection (not applicable)

VERIFICATION:
- TestSubnetDiscovery: ✅ PASS
- TestExtractSubnetIDs: ✅ PASS
- TestParseSubnetID: ✅ PASS
- Azure API mock tests: ✅ PASS

Both PRs now implement identical optimization logic while using appropriate Azure SDK versions.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

…subnet queries to eliminate NRP throttling

This PR implements a targeted subnet discovery optimization for Azure IPAM that eliminates Azure NRP throttling by replacing subscription-wide VNet enumeration with targeted subnet queries.

This PR is the same fix as cilium#41554 but ported to use Azure SDK v2.

Key Changes:

THREE-PHASE SUBNET DISCOVERY STRATEGY:
1. Query all node instances (existing method)
2. Extract unique subnet IDs from node network interfaces
3. Query only the specific subnets that nodes actually use

AZURE SDK V2 MIGRATION:
- Updated from SDK v1 to SDK v2 with armnetwork clients
- Added SubnetsClient for targeted subnet queries
- Implemented Pager pattern for pagination
- Added parseSubnetID function with regex validation

PERFORMANCE IMPROVEMENTS:
- Reduces API calls from O(n*m) to O(k) where k = unique subnets used by nodes
- Eliminates subscription-wide VNet enumeration that causes throttling
- Maintains backward compatibility with fallback to full VNet discovery

ALIGNED FEATURES WITH PR cilium#41554:
- Pre-compiled regex pattern with expectedCaptureGroups constant
- getSubnetWithPagination method for accurate IP configuration counting
- Enhanced parseSubnetID function with proper error handling
- Subscription ID tracking in Client struct
- Proper logrus-based logging in GetNodesSubnets
- Three-phase subnet discovery optimization

TEST RESULTS:
- All API tests pass including new parseSubnetID validation tests
- Azure IPAM functionality preserved with enhanced logging
- Existing IPAM allocation tests demonstrate continued functionality

BREAKING CHANGES:
None - maintains full backward compatibility with existing IPAM behavior.

PERFORMANCE IMPACT:
Significantly improved performance in large Azure environments with many VNets while maintaining the same IPAM functionality. The optimization shows "targeted_subnets" count in logs for visibility.

Signed-off-by: Cong Yue <yuecong1104@gmail.com>