stack.function search should index frames from top-level stacktrace (message events), not just exception frames

### Environment SaaS (https://sentry.io/) ### Steps to Reproduce 1. Send a message event (not an exception/crash) with a top-level stacktrace interface containing frames 2. In Discover, search using stack.function: for a function name present in that stacktrace 3. Alternatively, search the raw function name as a string ### Expected Result The event should be returned in search results, as the stack frames are visible in the issue UI. ### Actual Result No results returned. The stack.function field only indexes frames from exception.values[].stacktrace.frames, not from top-level stacktrace.frames. This means stack traces on message events are viewable but not searchable. ### Product Area Discover ### Link _No response_ ### DSN _No response_ ### Version _No response_