fix(blockrun): safely retry stale Solana blockhash

[jjcc123312]

Problem

Solana x402 signatures include a server-provided recent blockhash. A signed retry can be rejected with PAYMENT_INVALID / expired_signature or stale blockhash errors. The previous adapter treated every second 402 as terminal, so recoverable stale challenges failed; blindly retrying every 402 would risk duplicate settlement.

Scope and design

• Add a Solana-only retry state machine in relay/channel/blockrun.
• Retry only explicit verification-phase stale/blockhash errors.
• Fetch a fresh unsigned 402 challenge and sign a new payload for each retry.
• Bound retries to two attempts with 500ms and 2s backoff.
• Never retry SETTLEMENT_FAILED, insufficient_funds, ordinary 402 responses, or poll/settlement failures.
• Leave Base payment behavior and video channels unchanged.

Validation

• go test ./relay/channel/blockrun passes.
• Targeted BlockRun regression tests pass, including current OpenAI, nested Anthropic, legacy stale-blockhash, settlement, insufficient-funds, body restoration, and 64KB limits.
• go test ./relay/... reaches unrelated pre-existing failures in Claude/Codex packages; BlockRun packages pass.

Deployment

This changes shared relay request handling for BlockRun Solana and therefore requires router deployment after review. No staging or production deployment was performed by this change.