server: Report payment interval in payment challenges

[rickstaa]

Stacked on #4000 (which is stacked on #3999).

What does this pull request do?

Adds payment_interval_ms to the payment challenge JSON so paying clients know the orchestrator's debit cadence and can size and time payments to keep the session balance funded. Without it, clients have no way to learn the interval and risk being released for underpayment between top-ups.

Ports the challenge-interval change from #3998 by @ad-astra-video (originally approved as #3975), rebased onto the #3999/#4000 payment refactor. Credited as co-author.

Specific updates

• Add payment_interval_ms to liveRunnerPaymentChallengeResponse, populated from node.LivePaymentInterval in all three challenge producers: session reservation / single-shot challenge (runnerChallenge), scope challenge (scopePaymentChallenge), and the remote signer refresh path (RefreshPayment in rpc.go).
• Assert the field in TestLiveRunnerReserveSessionOnchainReturnsPaymentChallenge.
• Document the field in doc/live-runner.md.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 35.47615%. Comparing base (df527c3) to head (9e58323).

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #4001         +/-   ##
===================================================
- Coverage   35.53014%   35.47615%   -0.05399%     
===================================================
  Files            174         174                 
  Lines          45111       45112          +1     
===================================================
- Hits           16028       16004         -24     
- Misses         27814       27837         +23     
- Partials        1269        1271          +2     

Files with missing lines	Coverage Δ
server/ai_http.go	46.95652% <100.00000%> (+0.04196%)	⬆️
server/rpc.go	69.26714% <100.00000%> (ø)

... and 4 files with indirect coverage changes

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Files with missing lines	Coverage Δ
server/ai_http.go	46.95652% <100.00000%> (+0.04196%)	⬆️
server/rpc.go	69.26714% <100.00000%> (ø)

... and 4 files with indirect coverage changes

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[rickstaa]

Closing — the cadence this reports turns out not to be the number a paying client needs.

Both sides of the ledger bill by elapsed time at the same rate. The signer charges for the seconds since the last payment (server/remote_signer.go:476-490) and the orchestrator debits the seconds since its last tick (server/live_payment_processor.go:72-76, units × secSinceLastProcessed), using the same defaultSegInfo constant on both sides. So the client's cadence does not change what a session costs, only how much credit is banked at the moment AccountPayment runs its balance >= fee check.

Working that through, the safety condition is preroll ≥ cadence — the orchestrator's interval cancels out. A client paying every 3s is equally safe against -livePaymentInterval 1s and 300s; what would break it is a cadence above the signer's opening payment (10s for per-second pricing, 60s for pixel pricing).

So a client that read payment_interval_ms would not pick a different cadence with it than without it. Rather than ship a field that invites clients to size payments off the wrong bound, closing this.

The rebase onto master is done, so if this is wanted later it is a clean 14-line diff to reopen. Credit to @ad-astra-video, whose original this ports (#3998, approved as #3975) — the close is about what the client can do with the value, not the change itself.