🧳 fix: Carry Custom Endpoint Names Through Chat URLs (#15424)

`createPayload` interpolated the endpoint name straight into the chat URL,
so a custom endpoint named `Company/API` produced
`/api/agents/chat/Company/API`. The slash reads as a path separator, the
request no longer matches the router's `/:endpoint`, and it falls through
to the catch-all handler, which answers `404 Endpoint not found`.

Encoding the name keeps it a single path segment. Express matches the
encoded segment and decodes it back, and the endpoint the server acts on
comes from the request body regardless, so nothing else has to change.

Closes #15270