fix(rest): reject empty namespace separators

[fallintoplace]

Summary

• Reject empty REST namespace separators in namespace encode/decode helpers.
• Reject empty namespace separators when constructing ResourcePaths.
• Add regression coverage for direct helper calls and the resource path config boundary.

Why

DecodeNamespace advances by separator.size() after each match. An empty separator never advances the cursor, so a misconfigured separator can hang namespace parsing.

Testing

• cmake -S . -B build-rest -G Ninja -DICEBERG_BUILD_BUNDLE=OFF -DICEBERG_BUILD_REST=ON -DICEBERG_BUILD_HIVE=OFF -DICEBERG_BUILD_SQL_CATALOG=OFF
• cmake --build build-rest --target rest_catalog_test
• ctest --test-dir build-rest -R rest_catalog_test --output-on-failure

[wgtmac]

Could you please fix the ci failure and use conventional commit title for consistency?

[wgtmac]

Thank you, @fallintoplace!