fix(database): replace SQL JSON subscript with Python-side aggregation in pre-OTEL leaderboard

[Aftabbs]

Problem

get_leaderboard_aggregates() crashes with NotImplementedError whenever TRULENS_OTEL_TRACING is not enabled (the default for most users).

The root cause is in _get_leaderboard_aggregates_pre_otel: the SQL query uses SQLAlchemy's [] subscript operator to extract values from cost_json and references a non-existent Record.latency column:

sa.func.avg(self.orm.Record.cost_json["n_tokens"].as_float())  # raises NotImplementedError
sa.func.avg(self.orm.Record.latency)                            # AttributeError – column doesn't exist

cost_json and perf_json are declared as TYPE_JSON = Text (orm.py:35), so they are plain TEXT columns. SQLAlchemy raises NotImplementedError for [] subscript access on Text columns at statement construction time, before any query ever hits the database.

Fixes #2729.

Fix

Replace the broken SQL-level aggregation with Python-side aggregation using the existing module-level helpers _extract_tokens_and_cost and _extract_latency (already used by the get_records_and_feedback code path):

1. record_stmt now selects individual rows with raw cost_json and perf_json text columns (no subscript, no missing latency column).
2. After fetching, parse cost/latency with _extract_tokens_and_cost / _extract_latency.
3. Use pandas.groupby(...).agg(...) for the per-app aggregation that was previously done in SQL.

The output schema (Records, Total Tokens, Average Latency (s), Total Cost (USD)) is unchanged.

Tests

Added tests/unit/test_leaderboard_pre_otel.py with three test cases:

• empty database – returns empty DataFrame without raising.
• records present – aggregated Records, Total Tokens, Total Cost (USD), and Average Latency (s) are computed correctly.
• app_name filter – only the matching app's rows are returned.

All tests run against an in-memory SQLite database, matching the existing test pattern used in test_dashboard_utils.py.

Checklist

• Bug reproduced and root cause confirmed in orm.py (TYPE_JSON = Text)
• Fix avoids dialect-specific JSON extraction (works with SQLite, PostgreSQL, Snowflake)
• Output schema preserved
• Unit tests added

[joshreini1]

OTEL has been enabled by default for more than a year and its aggregation path already works. This PR should keep its scope on the legacy non-OTEL path, but implement that aggregation in SQL rather than moving it into frontend Python. SQL aggregation is materially more scalable and avoids transferring raw records to the frontend for processing.