feat(occurrences on eap): Implement EAP query for tagstore group tag value count

[shashjar]

Implements double reads of occurrences from EAP for get_group_tag_value_count in src/sentry/tagstore/snuba/backend.py.

[sentry]

Bug: The EAP query uses a fixed 90-day time window, while the control Snuba query uses a dynamic window, causing systematic undercounting for older groups.
_{Severity: HIGH}

Suggested Fix

Align the time windows between the control and experimental paths. The start and end datetime objects used for the main Snuba query should be passed to the count_occurrences call in the experimental path, replacing the hardcoded now - timedelta(days=90) value.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: src/sentry/tagstore/snuba/backend.py#L918-L919

Potential issue: The experimental EAP query path in `get_group_tag_value_count` uses a
hardcoded 90-day time window for its `count_occurrences` call. However, the control path
(the existing Snuba query) uses a dynamic time window that can be much larger, often
extending back to the group's `first_seen` date. This discrepancy causes the
experimental path to systematically undercount events for any group older than 90 days.
While this currently only affects experiment telemetry, it would lead to incorrect data
being displayed in production if this experimental code path were to be promoted.

_{Did we get this right? 👍 / 👎 to inform future reviews.}