fix: skip proof query when CreatedAtBlockHeight is 0 for old epochs

[mingles-agent]

Closes #983

Problem

GET /api/v1/epochs/{N}/participants returns 500 Internal Server Error for past epochs:

height must be greater than 0, but got 0

Repro: http://node1.gonka.ai:8000/api/v1/epochs/215/participants

Current epoch works fine. Past epochs (where data was stored before the CreatedAtBlockHeight field was introduced) fail.

Root Cause

In queryActiveParticipants (get_participants_handler.go):

1. First query (no height) fetches activeParticipants
2. blockHeight := activeParticipants.CreatedAtBlockHeight — for old epochs this is 0 (field not populated at the time of storage)
3. Second call QueryByKeyWithOptions(..., height=0, prove=true) — CometBFT rejects height=0

Fix

Check if blockHeight == 0 before the second query. If so, skip the proof query and return the first result directly. A Warn log is emitted for observability.

Testing

Verified against epoch 215 (previously returned 500) and epoch 216 (current, unaffected).

[Doog-bot534]

Review: fix: skip proof query when CreatedAtBlockHeight is 0 for old epochs

Approve ✅

Surgically correct fix. CometBFT rejects ABCI queries at height=0, so short-circuiting when CreatedAtBlockHeight == 0 prevents runtime errors for legacy epochs that predate the field. Returning the first query result without the proof query is the right fallback.

Strengths

• Correctly handles protobuf uint64 zero-value default as sentinel for old epochs
• Converts a hard failure into a graceful skip with appropriate Warn-level log
• Minimal regression risk since this path was already broken for old epochs

Suggestions

1. Include the epoch index in the log message for traceability: "epoch_index", epochIndex
2. Add a comment noting this guard can be removed once all CreatedAtBlockHeight == 0 epochs have aged out

Payout address: gonka10zaal553duxp05nvfpqtsqrm2g0j6j34r8nan7