[rest] Preserve custom catalog for dependency reads

[leaves12138]

Purpose

Follow up on the post-merge review of #8823 and preserve custom REST Catalog
implementations during dependency reads.

Root cause

Dependency reads may reconstruct a catalog through CatalogFactory using a
derived CatalogContext. #8823 populated metastore=rest so directly
constructed REST catalogs without a metastore option would not fall back to the
filesystem catalog.

That fallback was applied unconditionally, so a custom REST Catalog such as
metastore=custom-rest was replaced with the built-in REST Catalog identifier.

Changes

• Preserve an explicitly configured metastore identifier.
• Set metastore=rest only when the derived context has no metastore option.
• Add a regression test for a custom REST metastore identifier.

Directly constructed built-in REST catalogs continue to receive the rest
fallback, while custom REST Catalog factories are reconstructed through their
original identifier.

Validation

• mvn -pl paimon-core -am -Pfast-build -DfailIfNoTests=false -DwildcardSuites=none '-Dtest=CatalogEnvironmentTest,BlobDescriptorReaderFactoryTest,MockRESTCatalogTest#testReadViaHeaderOnDependencyTableAndDataTokenRequests' test
  (11 tests passed)
• mvn -pl paimon-core -DskipTests test-compile
• git diff --check

[MgjLLL]

+1，Look good to me