[docs] Clarify snapshot record count semantics

[wwj6591812]

Purpose

totalRecordCount and deltaRecordCount in snapshots are calculated from the row counts of data files. They are file-level, unmerged counts rather than logical table row counts.

This distinction is especially visible for Dedicated Format tables: appending N logical rows with one dedicated BLOB column produces N records in regular data files and N records in BLOB data files, so deltaRecordCount increases by 2 * N.

This PR preserves the existing file-level semantics discussed in #7779 and #7828. It clarifies the API and documentation instead of changing the counters to logical-row semantics.

Changes

• Clarify totalRecordCount and deltaRecordCount in the snapshot spec, system-table docs, PyPaimon docs, and Java API comments.
• Document the Dedicated BLOB example and recommend COUNT(*) when the logical row count is needed.
• Add regression assertions for one dedicated BLOB column (2 * N) and two dedicated BLOB columns (3 * N), while the existing reader assertions continue to verify N logical rows.

Tests

mvn -pl paimon-core -am -Pfast-build \
  -DfailIfNoTests=false \
  -Dsurefire.failIfNoSpecifiedTests=false \
  -DwildcardSuites=none \
  -Dtest=BlobTableTest#testBasic,MultipleBlobTableTest#testBasic package

Result: 2 tests passed, 0 failures/errors.

[wwj6591812]

@JingsongLi
hi,please cc, thx

[JingsongLi]

+1