feat: add batch_approve_milestones to crowdfund_vault

[Mayorlay]

Adds batch_approve_milestones to the crowdfund_vault Soroban contract, allowing maintainers to
approve multiple project milestones in a single transaction instead of one call per milestone.

Changes

src/errors.rs

• BatchTooLarge (32) — batch exceeds the 10-item cap
• EmptyBatch (33) — zero-item batch rejected upfront

src/events.rs

• BatchMilestoneDecision — input contracttype with project_id + milestone_id
• BatchMilestoneProcessedEvent — summary event emitted once per batch with approved_count

src/lib.rs

• batch_approve_milestones(env, admin, decisions) — new admin-only entry point
  • Guards: auth, paused, empty batch, oversized batch (max 10)
  • Per-item: skips missing projects and already-approved milestones silently
  • Emits existing MilestoneApprovedEvent per approved item (identifies each milestone ID)
  • Emits BatchMilestoneProcessedEvent summary with total approved count
  • Returns approved_count: u32

src/test.rs

• 6 new tests: approves_all, skips_missing_project, skips_already_approved, empty_batch_fails,
  oversized_batch_fails, non_admin_fails, paused_fails

Acceptance criteria

┌──────────────────────────────────────────┬───────────────┐
│ Criterion │ How satisfied │
├──────────────────────────────────────────┼───────────────────────────────────┤
│ Contract accepts a bounded batch payload │ BatchTooLarge guard at > 10 items │
├──────────────────────────────────────────┼────────────────────────────────────────────────┤
│ Per-item outcomes are deterministic │ Missing/already-approved items are always │
│ │ skipped; no partial-fail rollback │
├──────────────────────────────────────────┼────────────────────────────────────────────────┤
├───────────────────────────────────┼──────────────────────────────────────────────────────┤
│ Events identify processed │ MilestoneApprovedEvent { project_id, milestone_id } │
│ milestone IDs │ fired per approved item │
├─────────────────────────────────┼────────────────────────────────────────────────────────┤
│ Guards prevent oversized or │ EmptyBatch, BatchTooLarge, Unauthorized, │
│ inconsistent batches │ ContractPaused all checked before mutations │
└─────────────────────────────────┴────────────────────────────────────────────────────────┘

Testing

All existing tests remain unmodified. Run new tests with:

cargo test -p crowdfund_vault -- batch_approve_milestones

▸ Closes #869

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[Cedarich]

@Mayorlay please fix workfow