refactor(codegen): use text blocks for the static templates in both generators

The fbs-gen and proto-gen CodeGen builders emitted Java via long chains of
sb.append("...\n") calls. Convert the static multi-line templates (method
bodies, doc comments, builders) to Java text blocks with %-positional
.formatted(...) for the interpolated names — far more readable, the emitted
shape now mirrors the generated output line-for-line.

fbs-gen: 173 -> 79 appends, proto-gen: 191 -> 153 (the remainder are
single-line or loop-interleaved per-field emitters where a text block doesn't
fit). Generated output is byte-identical, verified by regenerating fbs + proto
and the fbs-gen/proto-gen unit tests.

Note: the decode-doc line keeps its sb.append form on purpose — as a text block
its `/// ... {@code ...}` line would trip the checkstyle rule that bans {@code}
in Markdown Javadoc (the rule scans raw source lines and can't tell it's a
string literal being emitted into generated code).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dfa1