fix(opengauss): preserve comments in table DDL

[openai0229]

Related issue

Closes #1874

Summary

Use openGauss's native pg_get_tabledef(table_oid) path for table DDL so table and column comments are preserved. Resolve the table OID with parameterized schema and table predicates, and remove only the leading session-level SET search_path statement from the displayed/exported DDL.

The SQL statement is defined in OpenGaussMetaDataConstants; OpenGaussMetaData only executes the constant and post-processes the returned DDL.

Affected surfaces

• Frontend / Web
• Backend / API / Storage
• Database plugin / Driver
• JCEF / Desktop packaging
• CI / Build / Release
• Documentation only

Verification

• Commands and results:
  • rtk mvn -pl chat2db-community-plugins/chat2db-community-opengauss -am -Dmaven.test.skip=false -DskipTests=false -Dtest=OpenGaussMetaDataTest -Dsurefire.failIfNoSpecifiedTests=false test from chat2db-community-server: 3 tests passed, 0 failures/errors/skips; reactor build successful.
  • rtk mvn -DskipTests package: all 48 Community backend modules built successfully.
• Manual verification: rebuilt and restarted the Community backend from this branch on port 10825; health request returned HTTP 200. The active Community storage does not contain an openGauss datasource, so no live Community UI/database call was claimed. Issue fix(opengauss): preserve comments in table View DDL #1874 contains the same-table openGauss 7.0.0-RC3 native-function evidence.
• UI evidence: N/A; database-plugin backend change.

Risk and compatibility

• Public API or stored data: N/A; no API contract or stored data changes.
• Database or driver compatibility: scoped to OpenGauss. It requires the native pg_get_tabledef(oid) overload. Native output can differ from the inherited PostgreSQL catalog reconstruction, including owner/grant statements; this PR intentionally follows the native OpenGauss DDL contract to preserve comments.
• Network, privacy, or security: no network behavior changes; schema and table values are bound as JDBC parameters.
• Community / Local / Pro boundary: Community plugin only. Enterprise is intentionally unchanged until the Community fix is reviewed and verified.
• Backward compatibility: other dialects are untouched. OpenGauss DDL output changes to native formatting and removes only a leading SET search_path ...; statement.

Reviewer map

• Start here: OpenGaussMetaData#tableDDL, then OpenGaussMetaDataConstants#TABLE_DDL_SQL and OpenGaussMetaDataTest.
• Failure condition: the target openGauss version lacks pg_get_tabledef(oid), schema/table resolution returns no row, or the native prefix format differs from a leading semicolon-terminated SET search_path statement.
• Rollback or disable path: revert commit 60da0482; OpenGauss will resume the inherited PostgreSQL DDL reconstruction path.

Contributor declaration

• I linked the Issue that defines this change.
• I tested the affected behavior and reported the actual results above.
• I did not include credentials, private data, or generated build output.
• I disclosed substantial AI assistance below, or this PR contains no substantial AI-generated code.

AI assistance: Substantial AI assistance was used for source tracing, implementation, test construction, verification, and PR drafting. The resulting diff and verification output were reviewed before submission.