Gate Sentry export by span level, add Level enums

[loks0n]

Summary

The span Sentry exporter previously sent every span carrying a Throwable and always reported it at error. This gates export on the span's level instead:

• Spans at warning or above are sent, each reported at its own level.
• A caught exception finished at Level::Warn now shows in Sentry as warning (not error).
• A caught exception downgraded to Level::Info/Level::Debug is now suppressed (previously any Throwable was sent).
• error/fatal behave as before.

Enums (no backward compat)

• Utopia\Span\Level — the library's vocabulary, following Grafana Loki's detected_level terms: debug / info / warn / error / fatal (note warn, not warning). This is the type accepted by Span::finish().
• Utopia\Span\Exporter\Sentry\Level — Sentry's own terms, with fromSpan() translating warn → Sentry's warning on the wire. Kept distinct so the core stays Loki-native and the Sentry contract lives in the exporter.

So a span finished at Level::Warn reads as warn in the Stdout/Pretty exporters and serializes to "level":"warning" in the Sentry envelope.

Notes

• The envelope is still exception-shaped: a warning span carrying no Throwable passes the gate but produces no event (no stacktrace to send). A captureMessage-style path for exception-less warnings could be added later if needed.
• Span::finish() signature changed from ?string $level to ?Level $level. The only in-repo caller (client) uses finish() with no level arg, so it's unaffected.

Testing

bin/monorepo check span (pint/phpstan/rector) and bin/monorepo test span (99 tests) pass.

🤖 Generated with Claude Code