[FLINK-39090][SQL] Enhance structured condition display of Explain output

[featzhang]

What is the purpose of the change

This PR enhances the explain output of FlinkLogicalJoin to display join conditions in a more readable and structured format. Instead of showing low-level positional references like =($0, $3), it now displays actual field names in a clear format.

Before:

Join(condition=[=($0, $3)])

After:

Join:
  Type: INNER
  Condition: user_id = id

Brief change log

• Add explainTerms override method in FlinkLogicalJoin class
• Implement formatJoinCondition method for recursive RexNode condition parsing
• Implement formatOperand method for field reference resolution
• Support for EQUALS, AND, OR operators with proper field name resolution
• Use FlinkRexUtil.getExpressionString as fallback for complex expressions

Verifying this change

This change is a display-only enhancement and can be verified by:

1. Build the flink-table-planner module:

   ./mvnw clean install -DskipTests -Pfast -pl flink-table/flink-table-planner

2. Run EXPLAIN queries in Flink SQL Client with joins:

   EXPLAIN SELECT * FROM orders o JOIN users u ON o.user_id = u.id;

3. Verify the join condition is displayed with field names instead of positional references

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): No
• The public API, i.e., is any changed class annotated with @Public(Evolving): No
• The serializers: No
• The runtime per-record code paths (performance sensitive): No
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: No
• The S3 file system connector: No

Documentation

• Does this pull request introduce a new feature? No (Enhancement only)
• If yes, how is the feature documented? Not applicable

This change only enhances the explain output format and does not modify any RelNode structure, query optimization logic, or runtime behavior. It has zero impact on performance and is 100% backward compatible.

[flinkbot]

CI report:

• da64458 Azure: SUCCESS

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