refactor(extensions): centralize validator operation names

[Sanderhoff-alt]

Summary

Centralizes the operation names passed through precheck, validate_bank_read, and validate_bank_write by adding PrecheckOperation, BankReadOperation, and BankWriteOperation StrEnum types. The existing PrecheckContext, BankReadContext, and BankWriteContext call sites now use enum members instead of scattered string literals.

Why this helps

ctx.operation is the main signal an OperationValidatorExtension can use for operation-specific authorization and quota checks. Before this change, the allowed values only existed as string literals spread across api/http.py, memory_engine.py, and HTTP route wiring, so extension authors had to search the codebase to discover the full set and could easily miss routes like bank config updates, async maintenance submissions, document/chunk access, file retain, or dry-run extraction.

Exported enums give extension authors a single source of truth, better IDE completion, and a clearer review surface when new operation-scoped hooks are added. They also make call sites self-documenting: BankWriteOperation.UPDATE_BANK_CONFIG and PrecheckOperation.FILES_RETAIN communicate the hook contract more clearly than raw string literals.

Compatibility

The new enums inherit from str, so existing extensions that compare ctx.operation to string values such as "retain", "get_bank_stats", or "update_bank_config" continue to work. The compatibility test covers both enum identity checks and legacy string comparison behavior.

Tests

• uv run pytest tests/test_extensions.py -q
• ./scripts/hooks/lint.sh

[nicoloboschi]

Reviewed for backwards compatibility.

The new PrecheckOperation/BankReadOperation/BankWriteOperation StrEnums are fully string-compatible — equality, str(), f-strings, format(), json.dumps, dict-key lookup, set membership, and str methods all behave as the bare string (verified empirically). The context dataclasses do no runtime type enforcement, so existing extensions comparing ctx.operation == "retain" and any not-yet-updated string call sites keep working. Exports are additive only; nothing removed.

Since this is a fork PR, test-api/Core LLM jobs are skipped in CI, so I ran tests/test_extensions.py locally on the PR branch: 42 passed.