feat: support START TRANSACTION and END TRANSACTION (#70)

[erans]

Summary

• Recognize START TRANSACTION as BEGIN and END/END TRANSACTION as COMMIT in QueryTypeDetector (both fast path and fallback)
• Add START/END to extended query protocol transaction dispatch and handler in extended.rs
• Enables postgres_fdw to connect (it sends START TRANSACTION ISOLATION LEVEL REPEATABLE READ)

Test Plan

• Unit tests for START TRANSACTION (upper, lower, mixed case, with isolation level clause)
• Unit tests for END / END TRANSACTION (upper, lower case)
• Word boundary guard tests (ENDLESS, ENDTABLE do NOT match)
• is_transaction() helper verifies all synonyms
• Full test suite passes (486 tests, 0 failures)
• cargo clippy — no new warnings

Closes #70

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