make termination reason serialization match valuable (#845)

The termination reason Valuable "serialization" differs from the serde
serialization. Let's make them match each other to avoid confusion.
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----

> [!IMPORTANT]
> Aligns `TerminationReason` serialization in
`plane/src/types/backend_state.rs` to match `valuable` and `serde`
formats.
> 
>   - **Behavior**:
> - Aligns `valuable::Valuable` serialization of `TerminationReason`
with `serde` serialization.
> - Changes string representations: `key_expired` to `keyexpired`,
`startup_timeout` to `startuptimeout`, `internal_error` to
`internalerror` in `TerminationReason`.
>   - **Files**:
> - Modifies `plane/src/types/backend_state.rs` to update `Valuable`
implementation for `TerminationReason`.
> 
> <sup>This description was created by </sup>[<img alt="Ellipsis"
src="https://img.shields.io/badge/Ellipsis-blue?color=175173">](https://www.ellipsis.dev?ref=jamsocket%2Fplane&utm_source=github&utm_medium=referral)<sup>
for 3e5fbd8. It will automatically
update as commits are pushed.</sup>

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Author: rolyatmax

plane/src/types/backend_state.rs

@@ -244,10 +244,10 @@ impl valuable::Valuable for TerminationReason {
         match self {
             TerminationReason::Swept => valuable::Value::String("swept"),
             TerminationReason::External => valuable::Value::String("external"),
-            TerminationReason::KeyExpired => valuable::Value::String("key_expired"),
+            TerminationReason::KeyExpired => valuable::Value::String("keyexpired"),
             TerminationReason::Lost => valuable::Value::String("lost"),
-            TerminationReason::StartupTimeout => valuable::Value::String("startup_timeout"),
-            TerminationReason::InternalError => valuable::Value::String("internal_error"),
+            TerminationReason::StartupTimeout => valuable::Value::String("startuptimeout"),
+            TerminationReason::InternalError => valuable::Value::String("internalerror"),
         }
     }