HogQL: tables under the posthog namespace are hard to discover

[jonmcwest]

Problem

Users who query a newer PostHog table without the posthog. prefix get Unknown table and no hint about the fix.

• SELECT * FROM logs works. SELECT * FROM trace_spans fails with Unknown table \trace_spans`. Only posthog.trace_spans` works.
• The cause is intended. Since feat(hogql): All new tables must be under the posthog namespace #42595, new tables live only under the posthog namespace. Tables that existed before stay in ROOT_TABLES__DO_NOT_ADD_ANY_MORE, so they work with or without the prefix.
• Discovery does not reflect this. By default, get_posthog_table_names() in posthog/hogql/database/database.py returns a hardcoded list: events, groups, persons, sessions, logs and the system tables. posthog.* tables only appear with include_hidden=True, so autocomplete and the sidebar never suggest posthog.trace_spans.

Tables affected include posthog.trace_spans, posthog.trace_attributes, posthog.metrics and posthog.ai_events.

Proposal

1. Better error. When an unknown table name matches a child of the posthog node, say Unknown table 'trace_spans'. Did you mean 'posthog.trace_spans'?. This covers every current and future namespaced table.
2. Better discovery. List the user-facing posthog.* tables (at least posthog.trace_spans and posthog.metrics) in the default get_posthog_table_names() output, so they appear in autocomplete and the sidebar.

This keeps the namespace rule from #42595 intact. It does not add anything to the root namespace.