Support MySQL LENGTH, BIT_LENGTH and LOCATE functions in SQL Federation

[Phixsura]

Background

SQL Federation rejects three common MySQL string functions because they are not registered in MySQLOperatorTable, producing No match found for function signature LENGTH/BIT_LENGTH/LOCATE(...). Reported in #31897, #31898, #31899.

Changes

• Add MySQLLengthFunction, MySQLBitLengthFunction, MySQLLocateFunction as SqlUserDefinedFunction subclasses with operand metadata matching MySQL semantics (UTF-8 byte count for character values, raw length for byte[], 1-based positions with the optional start position respected and clamped).
• Register LENGTH, BIT_LENGTH, LOCATE, and LOCATE_WITH_POSITION in MySQLOperatorTable.
• Calcite's SqlLibraryOperators.LENGTH lives in HIVE / POSTGRESQL / BIG_QUERY libraries and uses character-count semantics, so a MySQL-specific implementation is required.

Verification

• 30 new unit tests across the three functions covering null, empty, multi-byte UTF-8, byte[], and LOCATE position boundaries.
• 4 new cases in federation-query-sql-cases.xml pinning the generated Calcite plan; SQLStatementCompilerIT 111 / 111 (107 baseline + 4 new).
• spotless:check, checkstyle:check, apache-rat:check all green on kernel/sql-federation/compiler.

Closes #31897
Closes #31898
Closes #31899

[Phixsura]

Rebased onto latest master (now at 63b0fdbe). Previous CI on head 0a307e61 had two failures, both unrelated to this PR:

• CI job: PipelineJobProgressPersistServiceTest.assertPersistNowUpdatesProgressWhenJobItemContextPresentWithoutLogging failed with a Mockito UnfinishedStubbingException (cross-test stubbing leakage). The test lives in kernel/data-pipeline/core and is unrelated to the federation change here. I ran it locally on this branch (PipelineJobProgressPersistServiceTest: 9 tests, 0 failures, 0.8s).
• E2E - SQL (Smoke) (proxy, Cluster, MySQL, db_tbl_sql_federation): failed at step 3 "Download Source Snapshot" with Failed to ListArtifacts: (403) Forbidden. The federation tests never started; the failure is in artifact-download infrastructure.

Workflows are awaiting committer approval again after the rebase. Could a maintainer kindly approve them?