[FLINK-39613][runtime] Unquote schema metadata keys

[qiuyanjun888]

Summary

• Normalize primary-keys and partition-keys entries in SchemaMetadataTransform by stripping MySQL-style outer backticks.
• Preserve existing unquoted key behavior and unescape doubled backticks inside quoted identifiers.
• Add a focused regression test for backtick-quoted primary/partition key entries.

Root Cause

projection is parsed through Calcite with the MySQL dialect and therefore resolves backtick-quoted identifiers to their unquoted column names. In contrast, primary-keys and partition-keys were parsed with plain comma splitting and trimming, so entries like `order-id` were kept verbatim and did not match the schema column name order-id.

Fix

• Apply a small normalization step to primary/partition key entries after trimming.
• If an entry is enclosed in backticks, remove the outer backticks and decode doubled backticks (`` -> ).

Validation

• RED: mvn -pl flink-cdc-runtime -am -Dtest=org.apache.flink.cdc.runtime.operators.transform.SchemaMetadataTransformTest#testPrimaryAndPartitionKeysStripBackticks -DfailIfNoTests=false -DskipITs -Drat.skip=true -Dcheckstyle.skip=true -Dspotless.check.skip=true test
• GREEN: same focused test after the fix.
• mvn -pl flink-cdc-runtime -am -Dtest=org.apache.flink.cdc.runtime.operators.transform.SchemaMetadataTransformTest -DfailIfNoTests=false -DskipITs -Drat.skip=true -Dcheckstyle.skip=true -Dspotless.check.skip=true test
• mvn -pl flink-cdc-runtime -am -Dtest=org.apache.flink.cdc.runtime.operators.transform.PreTransformOperatorTest -DfailIfNoTests=false -DskipITs -Drat.skip=true -Dcheckstyle.skip=true -Dspotless.check.skip=true test
• mvn -pl flink-cdc-runtime -am -DskipTests -DskipITs -Drat.skip=true -Dcheckstyle.skip=true spotless:check
• mvn -pl flink-cdc-runtime -am -DskipTests -DskipITs -Drat.skip=true -Dspotless.check.skip=true test-compile

Jira

• https://issues.apache.org/jira/browse/FLINK-39613

Review

cc @yuxiqian @leonardBang, could you please take a look when you have time?

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