structured cache: fix some minor issues (#995)

- Fix double WriteResponse in PUT record failure path; the detail-body branch now short-circuits instead of falling through to a second WriteResponse call
- Return 405 Method Not Allowed for unsupported verbs in the root, namespace, bucket, record, and chunk handlers (previously fell through to no response)
- Clamp exec$/replay-recording thread_count so a bogus query value cannot spawn an unbounded worker pool

## Performance / cleanup

- NamespaceMap now uses TransparentStringHash + std::equal_to<>, so Get/Put/Find/Drop can probe the map with a std::string_view directly instead of constructing a temporary std::string on every request
- Replace insert_or_assign with try_emplace under the exclusive lock in GetNamespace; the find() re-check already guarantees the key is absent, so try_emplace matches intent better

## Reverted

- The earlier change to erase the pinned entry from m_DroppedNamespaces after DropNamespace's post-drop work was reverted: other threads may still hold pointers into a dropped namespace, so tearing it down eagerly is unsafe. Dropped namespaces remain pinned for the lifetime of the process as before.

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