feat: add AllocationCreated and AllocationCompleted events

[diptanu]

Summary

This PR refactors request state change events to clearly separate two distinct lifecycles, aligning with tensorlakeai/indexify PR #2042:

1. Function Run lifecycle - the logical unit of work (may have multiple execution attempts)
2. Allocation lifecycle - a single physical execution attempt on an executor

Changes

New Event Types

• AllocationCreated - Event emitted when an allocation (execution attempt) is created and assigned to an executor
• AllocationCompleted - Event emitted when an allocation (execution attempt) completes with an outcome

Modified Event Type

• FunctionRunCompleted - Now represents when a function run reaches its final outcome (after all retries exhausted or success). No longer contains allocation_id since it represents the logical function run, not a specific execution attempt.

Backward Compatibility

• FunctionRunAssigned kept as a type alias to AllocationCreated
• Legacy FunctionRunAssigned variant in the enum deserializes to AllocationCreated

Example Event Flow

With retry after executor removal:

RequestStarted(req-123)
├─ FunctionRunCreated(run-1, fn_a)
│  ├─ AllocationCreated(alloc-1, run-1, executor-A)
│  ├─ AllocationCompleted(alloc-1, failure/ExecutorRemoved)
│  ├─ AllocationCreated(alloc-2, run-1, executor-B)  ← retry
│  ├─ AllocationCompleted(alloc-2, success)
│  └─ FunctionRunCompleted(run-1, success)  ← final outcome
└─ RequestFinished(success)

Testing

• All existing tests pass
• cargo check passes