Fix budget reservation safety across storage backends

[Abiorh001]

Summary

• make budget reservations single-use and storage-atomic for memory, file, and Redis backends
• align BudgetGuard check/read paths with the active period counters
• remove old BudgetGuard reservation-token compatibility so unsupported token formats fail closed
• fix Redis numeric counter get/query behavior and exact decimal boundary checks for money comparisons
• update guard docs/examples and add focused regression tests for concurrency, idempotency, Redis, and scalar query handling

Verification

• uv run pre-commit run --all-files
• uv run pytest tests/test_guards.py::TestBudgetGuard tests/test_redis_storage.py tests/test_storage_query.py -q
• uv run pytest tests/test_guards.py::TestBudgetGuard tests/test_payment_concurrency.py tests/test_guard_edge_cases.py tests/test_sdk_events.py::TestBudgetGuardEvents tests/test_redis_storage.py tests/test_storage_query.py -q
• uv run pytest -q