fix(polymarket): retry bounded closed-lane Gamma discovery after 500

[proerror77]

Parent

#624

What to build

Make the Polymarket reference collector recover only from the observed closed-lane Gamma HTTP 500 by retrying the same bounded date window without the server-side crypto tag. Preserve the normal tagged query, the existing target-market admission checks, and fail-closed behavior for every other error.

Acceptance criteria

• A local Gamma test server proves that the tagged closed-lane 500 is retried through the existing bounded HTTP policy and then uses the untagged fallback with both original date bounds.
• The normal open and closed primary requests retain tag_id=21; a non-500 or failed fallback remains an error.
• The fallback admits only the existing supported-symbol, 5-minute/15-minute, two-token target markets.
• Focused collector tests and the applicable runtime/deployment contract test pass.

Blocked by

None - can start immediately.