Add Redos static analysis utility to labs.regex and RedosVulnerability Error Prone checker

[copybara-service]

Add Redos static analysis utility to labs.regex and RedosVulnerability Error Prone checker

Implements static AST analysis on RegexPattern to detect Regular Expression Denial of Service (ReDoS) / catastrophic backtracking vulnerabilities.

Features:

• Functional Stream-based and IntStream-based AST traversal returning ImmutableList with flatMap().
• Synthesizes the culprit attack witness input string and formula using StringFormat template and Substring.all().replaceAllFrom().
• Detects nested unbounded quantifiers, overlapping alternations, adjacent overlapping quantifiers, and optional element overlaps.
• Guards against false positives on possessive quantifiers, bounded limits, and disjoint delimiters.
• Adds Error Prone BugChecker RedosVulnerability extending AbstractPatternSyntaxChecker, generating RedosVulnerability_refactoring for flumejavac / JavacFlume.
• Comprehensive test coverage including unit tests, mutation testing (100% kill score), property-based AST and regex grammar fuzzer tests in RedosFuzzTest, and RedosVulnerabilityTest.
• Tests written with TestParameterInjector, Truth assertThat(), and JUnit4.