feat(server-utils): Emit low cardinality function.gcp span names for firebase functions

Firebase function spans were named `firebase.function.<triggerType>`, which does not follow the
FaaS span name convention. With span streaming enabled they are now named after the function,
falling back to `Serverless function execution` when neither `FUNCTION_TARGET` nor `K_SERVICE`
resolves. Static mode keeps the previous name.

`faas.name` no longer falls back to the literal string `unknown`, so that an unresolvable name
produces the static fallback rather than a span named `unknown`. The former span name is kept on
`gcp.function.context.type` so the span description can still be derived from it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: Lms24