fix(java-spring): add missing imports for converter (#23325)

[renatomameli]

closes #23325

The converter.mustache template uses {{{dataType}}} which resolves to short names like URI, UUID or BigDecimal for Java, but the template never imported those types.
A previous fix attempt in SpringCodegen.postProcessModelsEnum() tried to work around this by overwriting cm.dataType with the fully qualified name from importMapping (e.g. URI → java.net.URI). However, this approach pollutes the dataType globally — the FQN then also leaks into the enum model classes themselves, which is undesirable. An alternative would be to use vendor-extensions if desired.

But this PR follows the same pattern already established by model.mustache, which hardcodes commonly needed imports like java.net.URI and java.util.* (line 44) rather than dynamically resolving them. The fix:
converter.mustache: Add import java.net.URI; and import java.util.UUID; — the two non-java.lang types that can appear as enum dataType values.

Tests: Updated to assert that imports are present and short names are used in the converter.

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Summary by cubic

Adds missing imports to Spring converter.mustache so generated enum converters compile for URI, UUID, and numeric enums. Adds java.math.BigDecimal, java.net.URI, and java.util.UUID imports; updates samples and adds tests asserting the imports and short type names in EnumConverterConfiguration.java.

^{Written for commit 41aa7db. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 5 files