v0.12.237 — balance preflight cap (#217)

Fixed — huge default max_tokens no longer forces free-model fallback

• OpenClaw can send a model's default output ceiling as max_tokens (128k on Opus-class models). The low-balance preflight treated that as guaranteed spend, so a paid call to anthropic/claude-opus-5 could silently fall back to a free model even with $5+ in the wallet. The preflight now caps its assumed output at 4096 tokens via estimateBalancePreflightAmount(); the request payload is untouched and x402 still enforces the real server quote.
• estimateAmount() stays exact where worst-case matters: the strict maxCostPerRun cap and cached-balance deduction. The free-fallback path now also clears the stale paid estimate, so a request that fell back to free no longer deducts a paid-model estimate from the cached balance.
• Thanks to @0xCheetah1 for the fix and regression suite (#217).

---