[java] escape backslashes in escapeForJavadoc to stop comment breakout

[arib06]

What is the purpose of the change

SpecificCompiler.escapeForJavadoc embeds untrusted schema doc strings into generated Javadoc comments and neutralizes only a literal */, < and >. javac decodes Unicode escapes before it strips comments (JLS 3.3), so a doc value carrying the backslash-u escapes for the comment terminator is emitted verbatim and decoded to */ in the generated source, ending the Javadoc early and turning the rest of the doc into code. A field-level doc lands right before the field declaration, so the injected text becomes a clean class member.

The sibling escapeForJavaString (used for the SCHEMA$ literal) already doubles backslashes, so that path is safe; the Javadoc path was the gap. This applies the same neutralization at the single escaping helper, before the existing */ replacement, so no eligible Unicode escape can form. Valid docs without backslashes are emitted unchanged.

Verifying this change

This change added tests and can be verified as follows:

• Added TestSpecificCompiler#docCannotBreakOutViaUnicodeEscape, which compiles a record whose field doc contains a backslash-u escape for the comment terminator and asserts no eligible Unicode escape survives in any generated file. It fails on the pre-fix code and passes after.
• Existing docsAreEscaped_avro4053 and the tools golden-file codegen tests still pass (docs without backslashes are unaffected).

Documentation

• Does this pull request introduce a new feature? no