[cosn] FileSystem should be cached by authority

[thswlsqls]

Purpose

fix #8813

• HadoopCompliantFileIO kept a single fs field, so the first authority's FileSystem was returned for every path — multi-bucket access on one COSNFileIO was misrouted.
• Now caches per authority in a Map<String, FileSystem> keyed by path.toUri().getAuthority() (null → "DEFAULT").
• Mirrors merged [obs] FileSystem should be cached by authority #8589 (obs); paimon-oss-impl ([oss] FileSystem should be cached by authority #2417) and paimon-s3-impl ([fs] HadoopFileIO should cache FileSystem by scheme and authority #2504) already do the same.

Tests

• No unit test is feasible: createFileSystem() builds a real CosNFileSystem needing a COSN backend/credentials, and the module has no existing tests. Merged [obs] FileSystem should be cached by authority #8589 (obs), the identical sibling fix, also landed without tests.
• mvn -pl paimon-filesystems/paimon-cosn-impl clean install passed.

[JingsongLi]

+1