[Bug][Connector-V2][Postgres-CDC] Handle duplicate replication slot by SQL state

[LeonYoah]

Purpose of this pull request

Fixes #10992.

Postgres CDC should not depend on a localized error message to detect an existing replication slot. This PR keeps the SQLState 42710 check and moves it into a helper, with the old English message check as a fallback.

Basis:

• PostgreSQL documents 42710 as duplicate_object in PG 13 and current PG.
• PostgreSQL replication slot creation reports a duplicate slot name with ERRCODE_DUPLICATE_OBJECT: slot.c.

Does this PR introduce any user-facing change?

Yes. When the replication slot already exists and PostgreSQL returns a localized message, Postgres CDC logs a warning and continues.

How was this patch tested?

JAVA_HOME=/Library/Java/JavaVirtualMachines/temurin-8.jdk/Contents/Home ./mvnw -pl seatunnel-connectors-v2/connector-cdc/connector-cdc-postgres -am -DskipITs -Dcheckstyle.skip -Dspotless.check.skip=true -Dlicense.skip=true -DskipTests install

JAVA_HOME=/Library/Java/JavaVirtualMachines/temurin-8.jdk/Contents/Home ./mvnw -pl seatunnel-connectors-v2/connector-cdc/connector-cdc-postgres -DskipITs -Dcheckstyle.skip -Dspotless.check.skip=true -Dlicense.skip=true -Dtest=PostgresSourceFetchTaskContextTest test

JAVA_HOME=/Library/Java/JavaVirtualMachines/temurin-8.jdk/Contents/Home ./mvnw -pl seatunnel-connectors-v2/connector-cdc/connector-cdc-postgres -DskipITs -Dcheckstyle.skip -Dlicense.skip=true spotless:check
git diff --check

Local reproduction:

• PG 13 zh_CN returned sqlState=42710 and message 错误: 复制槽名 "seatunnel" 已经存在.
• Before the fix, SeaTunnel failed at PostgresSourceFetchTaskContext.
• After the fix, SeaTunnel logged the existing-slot warning, entered incremental mode, and Console received INSERT : 4, delta_fix.
• PG 18.3 also returned sqlState=42710 for a duplicated replication slot.

Check list

• If any new Jar binary package adding in your PR, please add License Notice according
  New License Guide
• If necessary, please update the documentation to describe the new feature. https://github.com/apache/seatunnel/tree/dev/docs
• If necessary, please update incompatible-changes.md to describe the incompatibility caused by this PR.
• If you are contributing the connector code, please check that the following files are updated:
  1. Update plugin-mapping.properties and add new connector information in it
  2. Update the pom file of seatunnel-dist
  3. Add ci label in label-scope-conf
  4. Add e2e testcase in seatunnel-e2e
  5. Update connector plugin_config