Adds a RequestMeta context bridge so events dispatched during an HTTP
request can be attributed to it: a middleware stashes IP/UA/request-id
on the request context, the generic Do* handlers associate that context
with the transaction key, and DispatchPending/DispatchWithContext copy
the metadata onto the watermill message at publish time. Existing
dispatch call sites are unchanged.
Keeps the Do* helpers framework-neutral so non-Echo callers (upcoming
Huma /v2 handlers) don't need a translation shim.
Addresses review feedback on #2670.
kolaente