`exists_filter` on coalesced attributes misses v1 spans stored under deprecated keys

[buenaflor]

Environment

• Data shape: legacy v1 transaction spans where attributes are stored under deprecated keys
• Query path: user-facing has: syntax in Sentry (exists_filter)

Steps to Reproduce

1. Use v1 transaction span data where only deprecated keys are populated for a coalesced stable attribute (canonical key absent), e.g. http.response_content_length present and http.response.body.size missing.
2. Run an existence check on the canonical key, e.g. has:http.response.body.size (RPC exists_filter).

Expected Result

exists_filter on the canonical key should match when the value exists under a mapped deprecated key (consistent with canonical value-filter behavior).

Actual Result

Canonical value comparison matches, but canonical exists_filter does not match rows where only deprecated-key storage exists (observed on v1 spans).

Dependents

Currently blocks migration of Sentry dashboards that need consistent filtering across both v1 and v2 spans.

[linear-code]

EAP-475