Clean up low-severity CodeQL code-quality alerts

[vharseko]

Resolves a batch of CodeQL note-level (code-quality) alerts. No behaviour changes; all touched Java modules compile.

Commit 1 — 17 alerts across 15 files

• inefficient-empty-string-test (8): use String.isEmpty() instead of "".equals(...)
• inefficient-string-constructor (2): drop redundant new String(...) wrappers
• inefficient-key-set-iterator (2): iterate entrySet() instead of keySet() + get()
• useless-tostring-call (2): remove toString() on values already String
• local-variable-is-never-read (3): remove dead local assignments

Commit 2 — 7 alerts across 5 files

• java/local-shadows-field (5): rename local variables that shadowed fields in CsvAuditEventHandler, JsonFileWriter, and WarningHeader
• java/call-to-object-tostring (1): log VersionedPath.getVersions() instead of the object's default toString() in CrestApiProducer
• py/unused-import (1): drop the unused import sys, os in the persistit doc conf.py

Not addressed here (deliberate)

• java/class-name-matches-super-class (2): renaming these public classes would break the API — dismissed as won't-fix
• java/local-variable-is-never-read (1): BufferPool keeps a 1 MB reserve buffer released on OutOfMemoryError; the "unused" write is intentional — dismissed as won't-fix
• java/chained-type-tests (2) in Converter.java left for a separate, dedicated refactor