fix: bind isBatch on the legacy translate endpoint under Jackson 3

The public legacy endpoint POST /v2/public/translator/translate deserializes into
LegacyTolgeeTranslateRequest, whose `isBatch: Boolean` field silently stopped binding
after the Spring Boot 4 upgrade moved the JSON stack from Jackson 2 to Jackson 3
(tools.jackson). Requests now fail with request_parse_error (HTTP 400):

    Missing required creator property 'isBatch' (index 6)

Cause: Kotlin compiles a `val isBatch: Boolean` property to the JVM getter `isBatch()`
(the `is`-prefixed form, not `getIsBatch()`). Jackson derives a property's external JSON
name from its getter and, for an `is`-getter returning boolean, strips the prefix:
`isBatch()` -> external name `batch`. Jackson 2 + jackson-module-kotlin kept the external
name aligned to the Kotlin property name (`isBatch`), so incoming `"isBatch"` bound. Under
Jackson 3 the boolean `is`-getter naming wins, so the field binds to the JSON key `batch`
instead; the incoming `"isBatch"` no longer matches, and because the parameter is non-null
with no default it is reported as a missing required creator property. The implicit name in
the error is still `isBatch` (the constructor parameter name) even though the key Jackson was
actually looking for is `batch`.

Fix:
- @JsonProperty("isBatch") pins the external name back to `isBatch` so real clients (and the
  SDK/CLI) that send `"isBatch"` bind correctly again, including `isBatch = true`.
- `= false` default makes the field optional so requests that omit it do not hit
  "missing required creator property" regardless of the resolved key name, matching the
  pre-upgrade behavior where a missing boolean defaulted to false.

Surfaced by the billing EeTolgeeTranslatorControllerTest suite, which exercises this endpoint.

Author: bdshadow

ee/backend/app/src/main/kotlin/io/tolgee/ee/data/LegacyTolgeeTranslateRequest.kt

@@ -1,5 +1,6 @@
 package io.tolgee.ee.data
 
+import com.fasterxml.jackson.annotation.JsonProperty
 import io.tolgee.model.mtServiceConfig.Formality
 
 data class LegacyTolgeeTranslateRequest(
@@ -9,7 +10,8 @@ data class LegacyTolgeeTranslateRequest(
   val targetTag: String,
   val metadata: Metadata?,
   val formality: Formality?,
-  val isBatch: Boolean,
+  @JsonProperty("isBatch")
+  val isBatch: Boolean = false,
   val pluralForms: Map<String, String>? = null,
   val pluralFormExamples: Map<String, String>? = null,
 ) {