Add disk-aware EC rebalancing (#7597)

* Add placement package for EC shard placement logic

- Consolidate EC shard placement algorithm for reuse across shell and worker tasks
- Support multi-pass selection: racks, then servers, then disks
- Include proper spread verification and scoring functions
- Comprehensive test coverage for various cluster topologies

* Make ec.balance disk-aware for multi-disk servers

- Add EcDisk struct to track individual disks on volume servers
- Update EcNode to maintain per-disk shard distribution
- Parse disk_id from EC shard information during topology collection
- Implement pickBestDiskOnNode() for selecting best disk per shard
- Add diskDistributionScore() for tie-breaking node selection
- Update all move operations to specify target disk in RPC calls
- Improves shard balance within multi-disk servers, not just across servers

* Use placement package in EC detection for consistent disk-level placement

- Replace custom EC disk selection logic with shared placement package
- Convert topology DiskInfo to placement.DiskCandidate format
- Use SelectDestinations() for multi-rack/server/disk spreading
- Convert placement results back to topology DiskInfo for task creation
- Ensures EC detection uses same placement logic as shell commands

* Make volume server evacuation disk-aware

- Use pickBestDiskOnNode() when selecting evacuation target disk
- Specify target disk in evacuation RPC requests
- Maintains balanced disk distribution during server evacuations

* Rename PlacementConfig to PlacementRequest for clarity

PlacementRequest better reflects that this is a request for placement
rather than a configuration object. This improves API semantics.

* Rename DefaultConfig to DefaultPlacementRequest

Aligns with the PlacementRequest type naming for consistency

* Address review comments from Gemini and CodeRabbit

Fix HIGH issues:
- Fix empty disk discovery: Now discovers all disks from VolumeInfos,
  not just from EC shards. This ensures disks without EC shards are
  still considered for placement.
- Fix EC shard count calculation in detection.go: Now correctly filters
  by DiskId and sums actual shard counts using ShardBits.ShardIdCount()
  instead of just counting EcShardInfo entries.

Fix MEDIUM issues:
- Add disk ID to evacuation log messages for consistency with other logging
- Remove unused serverToDisks variable in placement.go
- Fix comment that incorrectly said 'ascending' when sorting is 'descending'

* add ec tests

* Update ec-integration-tests.yml

* Update ec_integration_test.go

* Fix EC integration tests CI: build weed binary and update actions

- Add 'Build weed binary' step before running tests
- Update actions/setup-go from v4 to v6 (Node20 compatibility)
- Update actions/checkout from v2 to v4 (Node20 compatibility)
- Move working-directory to test step only

* Add disk-aware EC rebalancing integration tests

- Add TestDiskAwareECRebalancing test with multi-disk cluster setup
- Test EC encode with disk awareness (shows disk ID in output)
- Test EC balance with disk-level shard distribution
- Add helper functions for disk-level verification:
  - startMultiDiskCluster: 3 servers x 4 disks each
  - countShardsPerDisk: track shards per disk per server
  - calculateDiskShardVariance: measure distribution balance
- Verify no single disk is overloaded with shards

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