Use fallback preconfer when lookahead is slashed

[linoscope]

In our current implementation, when an invalid lookahead is slashed, the invalid lookahead is kept as is. This is fine as long as the slashed amount (which can be full amount as invalid lookahead is fully attributable) exceeds the potential revenue of pushing invalid lookahead and sequencing for a whole epoch.

To be more robust, we can fallback to a fallback preconfer when an invalid lookahead is slashed.

[dantaik]

@linoscope triage check 👋 — trying to move this one forward.

Falling back to a fallback preconfer when an invalid lookahead is slashed — is this still planned as part of the post-whitelist preconf work?

Note this looks closely related to #20652 (override lookahead with fallback preconfer at invalid-lookahead slashing) — if they're the same thing we could consolidate into one. Keep open / close / dedupe?