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_

Start by tracing how Discover indexes stack.function from exception frames, then compare that path with the top-level stacktrace.frames used by message events. Reproduce the reported message event and verify that searching by the frame's function name and as a raw string returns the event.

Written by the indexing model from the issue text.