[ferenc-csaky]

What does this change?

Removes the flexible-csv format. Its only addition over Flink's csv format was the skip-header option. DataSQRL no longer infers the schema of CSV files, and it copies data files unchanged (DataSQRL/sqrl#2476), so CSV sources declare their columns and their files have no header row. Without skip-header, flexible-csv is the same as csv.

• Removed: the formats/flexible-csv-format module, including FlexibleCsv and the per-file header skipping from fix(flexible-csv): Skip the CSV header once per file instead of once per split #417 (FlexibleCsvFileFormat, FlexibleCsvBulkFormat) with their tests. Also its dependency in flink-sql-runner/pom.xml and its entry in FactoryAutoServiceTest.
• README.md: removes the CSV Format section and the "Additional configuration options for CSV format" feature bullet. CLAUDE.md no longer lists the module.

Breaking change

Tables with 'format' = 'flexible-csv' fail with Flink's Could not find any factory for identifier 'flexible-csv' that implements '...DeserializationFormatFactory' in the classpath.. To migrate:

1. Switch to 'format' = 'csv'.
2. Rename flexible-csv.* options to csv.*.
3. Drop skip-header, and remove header rows from the data.

This also removes the old Kafka behaviour, where skip-header = true by default dropped the first record each reader decoded.

Area

• Runner CLI and Docker image (flink-sql-runner)
• Connectors (connectors)
• Formats (formats)
• Function libraries (stdlib)
• Environment utilities (env-utils)
• Tests and samples (testing)
• Build / CI / dependencies
• Docs (README.md, stdlib-docs)

How did you test it?

• mvn compile -pl flink-sql-runner -am passes, so nothing else referenced the module.
• FactoryAutoServiceTest passes.
• mvn spotless:check passes.
• git grep -i flexible-csv finds nothing.

Checklist

• The PR title starts with a type such as feat:, fix: or docs:, optionally with a scope (e.g. fix(flexible-json):)
• mvn clean install passes locally and the code is formatted with mvn spotless:apply (formatted; ran only the builds above, with -Dlicense.skip=true because the license plugin can't read a git worktree)
• New or changed behavior is covered by tests (given_when_then naming, AssertJ assertions) (removal only)
• New dependency and plugin versions are defined as properties in the root pom.xml (no new dependencies)
• Commits are signed off and signed (git commit -s -S)
• README.md and stdlib-docs are updated if user-facing behavior or functions changed

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