Streaming output guardrails: support strict (400) and lenient (topic-change) rejection modes

[coderabbitai]

Summary

Currently, apply_stream_output_guardrails in src/proxy/handlers/format_handler.rs only supports a single hard-rejection path that returns a GatewayError::Validation error. Error handling should be extended to support two configurable modes:

1. Strict mode – return a 400 Bad Request status code when the guardrail blocks streaming output.
2. Lenient mode – instead of an error, return a simulated "Let's change the subject" (topic-deflection) response, giving a softer guardrail experience.

Background

Raised during review of PR #108 (comment: api7/aisix#108 (comment)) as a follow-up to the streaming output guardrail implementation.

Requested by

@bzp2010

[coderabbitai]

🔗 Related PRs

api7/aisix#23 - feat(provider): add stream bridged and native implementations [merged]
api7/aisix#25 - feat(provider): add llm sdk entrypoint for streaming [merged]
api7/aisix#27 - feat(provider): add anthropic chat format for streaming [merged]
api7/aisix#92 - feat: unified chat handler [merged]
api7/aisix#104 - feat(guardrail): integrate into chat handler [merged]

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