feat(solana): poll media jobs with a wallet ownership proof, not a payment

[VickyXAI]

Implements the sol.blockrun.ai poll-ownership contract (live since rev 00561-ghx; server source blockrun-sol/src/lib/poll-ownership.ts).

What changes (Solana wallet rail only)

• Media polls send x-poll-wallet / x-poll-timestamp / x-poll-signature: an ed25519 signature by the submitting wallet over blockrun-poll:v1:<video|audio|image>:<job id>:<unix seconds>, signed fresh per poll. The job id is the URL-decoded last segment of poll_url. Uses the solders Keypair behind the existing x402 signer, so there's no new dependency.
• Video: pending polls carry only the proof and never call the payment signer. The finished-but-unpaid poll answers 402 with the challenge and status:"completed". The client signs one payment from that challenge (terms pinned to the submit's) and re-sends the GET. If the paid poll gets a 402 (concurrent claim or stale blockhash), the client goes back to proof polling, which returns already_settled or a fresh challenge. Payments are capped at 1 + max_resigns.
• Images and music: these were paid at POST, so their polls never pay. A batch-billed image 402 falls back to the payment path.
• Fallback: if the gateway refuses the proof (video 400 Invalid job id, audio/image 404 Job not found), the client falls back once, for that job, to the existing payment-signature polling. This is logged as a warning on blockrun_llm.solana_client.
• Fix: slow music() tracks on the Solana wallet rail are now polled. Before this, the 202 stub was returned as the result and failed MusicResponse validation after the track had been paid for.
• Unchanged: the account rail (API key), Base clients (jobs.py, music.py, video.py, image.py), and any signer without a keypair.

The poll loop moved from _request_image_with_payment into _poll_media_job (sync and async) so music can share it. The legacy branch is the old code, kept verbatim.

Tests

• tests/unit/test_solana_poll_ownership.py (34 tests). Signatures are checked with an independent pure-Python RFC 8032 verifier and base58 decoder. The fake gateways enforce the contract.
• They cover: exact message bytes; kind/job id parsing; pending video polls with no signer calls; completion 402 leading to exactly one payment and then a 200; in-flight settlement not paying twice; capped completion payments; a repriced completion challenge being refused; image and music polls never paying; batch 402 fallback; refusal fallback (including another wallet's proof); account rail and Base music never sending a proof. Each case runs sync and async.
• Planted defects all went red, then were reverted: v1→v2 message (11 failed), completion signing the submit challenge (sync 3 / async 1), proof polls also carrying the payment header (11), fallback disabled (5), job id not URL-decoded (7), uncapped completion payments (1).
• black --check ., ruff check . and pytest tests/unit all pass (1152 passed). No new mypy errors in the touched files. No version bump, no publish, and the paid API was not called.