fix: use GCS generation precondition for Firstrade claims

[Pigbibi]

Why

The live execution claim introduced in PR 249 calls an object-store create_text method that is not in the pinned QuantPlatformKit contract. This fails closed before order submission, but prevents valid live cycles.

Change

• use GCS if_generation_match=0 directly for create-only claim writes
• return false only when the claim already exists
• retain fail-closed behavior for other storage errors

Verification

• targeted strategy-claim and rebalance tests passed
• git diff whitespace check passed