dex: fix chain halt in testnet 68

Testnet 68 halted at height 100736, with an `.expect` here:

https://github.com/penumbra-zone/penumbra/blob/1c99e24ad5cf1ecc2855849d66221ecec25f9235/crates/core/component/dex/src/component/dex.rs#L66

This hit an error bubbled up from here:

https://github.com/penumbra-zone/penumbra/blob/1c99e24ad5cf1ecc2855849d66221ecec25f9235/crates/core/component/dex/src/component/router/route_and_fill.rs#L277

The error occurs in this method, which is only ever used at that callsite:

https://github.com/penumbra-zone/penumbra/blob/1c99e24ad5cf1ecc2855849d66221ecec25f9235/crates/core/component/dex/src/swap_execution.rs#L18

It's a little unclear why that method has double fallibility. Unfortunately, the answer may not be easily determined. It was added here

9cd566d

which indicates that there was previously an infallible `max_price`, but that code isn't included in the commit; the previous reference to an infallible `max_price` was added in this commit

b4b2635

which doesn't have the impl either, so presumably it got mangled during rebasing.

In any case, removing the double fallibility, as in this commit, allows committing block 100736 on testnet 68, which I verified by running this code against a local copy of a state snapshot.

crates/core/component/dex/src/component/router/route_and_fill.rs

@@ -274,7 +274,7 @@ pub trait RouteAndFill: StateWrite + Sized {
             }
 
             // Ensure that we've actually executed, or else bail out.
-            let Some(accurate_max_price) = execution.max_price()? else {
+            let Some(accurate_max_price) = execution.max_price() else {
                 tracing::debug!("no traces in execution, exiting route_and_fill");
                 break;
             };

crates/core/component/dex/src/swap_execution.rs

@@ -15,17 +15,17 @@ pub struct SwapExecution {
 
 impl SwapExecution {
     /// Returns the price of the latest execution trace.
-    pub fn max_price(&self) -> Result<Option<U128x128>> {
+    pub fn max_price(&self) -> Option<U128x128> {
         let Some((input, output)) = self.traces.last().and_then(|trace| {
             let input = trace.first()?;
             let output = trace.last()?;
             Some((input, output))
         }) else {
-            return Ok(None);
+            return None;
         };
 
-        let price = U128x128::ratio(input.amount, output.amount)?;
-        Ok(Some(price))
+        let price = U128x128::ratio(input.amount, output.amount).ok()?;
+        Some(price)
     }
 }