[catalog] convert mz_audit_events to BuiltinMaterializedView

[mtabebe]

Problem:
mz_catalog.mz_audit_events is a BuiltinTable. This requires coordinated
writes from a single environmentd process, conflicting with the
multi-envd goal. The table also carried a special-case bootstrap path
(`bootstrap_audit_log_table` in `coord.rs`) that diffed the in-memory
`AuditLogIterator` against the table contents on every boot.

Solution:
Convert mz_audit_events to a BuiltinMaterializedView backed by a query
over mz_internal.mz_catalog_raw, and delete the bootstrap path.

Each AuditLog row in the durable catalog has a `proto::AuditLogKey`
whose `event_type` and `object_type` are numeric `Serialize_repr` codes
that the MV maps back to the kebab-case strings produced by the prior
populator (`audit_log::{EventType,ObjectType}` Display). `details` goes
through a new SQL helper:
  mz_internal.parse_catalog_audit_log_details(jsonb) -> jsonb

Breaking change:
The `mz_audit_events.details` JSON shape changes for variants where the
proto and audit-log structs diverge beyond `#[serde(flatten)]` /
`StringWrapper`.
queries:

Testing:
- New `src/expr/src/scalar/func/impls/jsonb.rs#tests` Rust unit tests
  cover variant strip, flatten, recursive flatten, StringWrapper
  unwrap, null option, non-FullNameV1 negative cases, and error inputs.
- test/sqllogictest/mz_audit_events.slt new lockdown test (schema,
  OID stability, item type, NOT-NULL invariants, monotonicity, every
  reachable (event_type, object_type) pair, representative details
  extractions, NULL user, deterministic occurred_at, PUBLIC_SELECT).
- test/sqllogictest/singlereplica_audit_log.slt updated for new shape.
- test/sqllogictest/{information_schema_tables,oid,materialized_views,
  cluster,catalog_server_explain,mz_catalog_server_index_accounting,
  rbac_mcp_agent,rename,transform/predicate_pushdown,
  autogenerated/mz_catalog}.slt all pass.
- test/testdrive/{catalog,audit-log}.td updated for new shape.