Drop rate-limited static invoice requests

[tnull]

Drop static invoice requests when the rate limit is reached instead of retrying them and delaying later events. Keep retrying storage errors and only acknowledge invoices after saving them.

Fixes an issue reported by @ajaysehwal.

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[ajaysehwal]

@tnull , one gap in thorough test suite: there’s no test covering a sender that gets rate-limited once and then successfully retries after the bucket refills.

Not required for correctness — the existing tests already show that the rate-limited request is dropped cleanly without affecting other requests. But a small test using a short refill interval or deterministic bucket advancement would make the intended contract explicit: the request is dropped, and the sender is free to retry later rather than being blocked permanently.

Purely a documentation-via-test nice-to-have.

[ajaysehwal]

LGTM