Add AUTO mode gates for prompt repetition (PR2)

[qicesun]

What

• Add PromptRepetitionMode.AUTO
• Add maxChars gate
• Add reasoningKeywords gate
• Add builder config for PromptRepetitionPolicy
• Keep idempotence robust for separator-heavy inputs

Behavior

• AUTO: skip too long and reasoning-intent prompts
• ALWAYS: still repeats unless already repeated
• Input guardrail still skips RAG-augmented input by default

Tests

• Positive/negative/edge/corner coverage for all AUTO gates
• Gate order checks (ALREADY_REPEATED before other AUTO gates)
• Case-insensitive keyword detection and maxChars boundary checks
• Randomized idempotence stress checks
• ./mvnw -pl langchain4j-community-core verify -DskipITs

Depends on #571
Related #570

[qicesun]

Quick update: I opened #592 as the new draft PR that carries prompt repetition forward in the dedicated langchain4j-community-prompt-repetition module requested during review.

I also found an upstream langchain4j bug in AiServices input-guardrail sequencing:

• bug: [BUG] AiServices input guardrails run before content arguments are materialized into the final UserMessage langchain4j#4678
• draft fix PR: fix: materialize content arguments before input guardrails langchain4j#4679

Because of that, #592 is now the main review target and this PR is effectively superseded by the dedicated-module draft plus the upstream fix.