Address all static analysis findings from internal import

Fix all static analysis findings identified during internal import across
12 affected files in Gitiles:

- Documentation: Add missing Javadoc comments to public classes,
  interfaces, builder classes, and enums.
- Language & Syntax Modernization: Convert multiline strings to Java text
  blocks, convert statement switches to arrow expression switches,
  adopt pattern matching for instanceof and switches, and group method
  overloads together without interleaving helper methods.
- Deprecation Resolutions: Migrate from deprecated PersonIdent constructor
  to Instant/ZoneId variants, replace OrderedList.getStartNumber() with
  getMarkerStartNumber(), and replace Splitter.onPattern() with
  Splitter.on(Pattern.compile(...)).
- Guava & Truth Assertions: Adopt ImmutableList/ImmutableSet where
  appropriate, use buildOrThrow(), use computeIfAbsent(), and modernize
  Truth assertions with isPresent(), hasSize(), and isEmpty().
- Style & Cleanliness: Enforce lowerCamelCase identifier names, bracket
  single-line control flow statements, split multi-variable declarations,
  avoid Yoda conditions, and make utility class constructor private.

Change-Id: Ia76966182374aef1c8cf6fb8d207567f4615c8f1