askrene: fixed a timeout corner case

[Lagrang3]

We've introduced a time out protection in #8636 to avoid hitting infinite loops
in askrene.

I've found a test case that hits the time out, which we don't usually want. We should be smart
enough to detect deadlocks and give up with something like "Payment is unfeasible".

To reproduce the condition we need two channels the first with an HTLC max = x
and the second with a base fee = x. So that any amount f to forward to the 3rd
node requires at least x+f to pass through the first channel, which is not
possible due to the HTLC max limit.
Refine removes all sats from the route and linear routes' while loop continues
coming back to the exact same situation at each iteration. We only end this
timing out.

We fix the infinite loop by deciding (under some arbitrary condition)
when a path has a bottleneck and then disabling one channel in that path that causes the bottleneck.

Important

26.04 FREEZE March 11th: Non-bugfix PRs not ready by this date will wait for 26.06.

RC1 is scheduled on March 23rd

The final release is scheduled for April 15th.

Checklist

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