fix(core,sdk,webapp): allow 10 session trigger tags, matching the run tag limit (#4832)

## Summary

`SessionTriggerConfig.tags` was capped at 5, while runs (and the [tags
docs](https://trigger.dev/docs/tags)) allow 10. Session trigger tags are
forwarded verbatim as the run tags on every run a session schedules, so
the lower cap was an inconsistency rather than a separate limit. For
`chat.agent` it was worse in practice: the SDK prepends `chat:{chatId}`
automatically and truncates, so users could only get 4 of their own tags
through.

The schema, the SDK truncation points, and the dashboard playground now
all use 10. `chat.agent` users get 9 of their own tags plus the
automatic `chat:{chatId}` tag. Docs updated to say so.