fix: keep agent prompt cache prefix stable

[danny-avila]

Summary

• split AgentContext system content into a stable cacheable prefix and dynamic additional_instructions tail
• keep Anthropic cache_control only on stable system text and place Bedrock cache points before dynamic text
• treat cross-run summaries as dynamic system tail while preserving mid-run compaction as HumanMessage
• add opt-in live Anthropic and Bedrock prompt-cache tests that verify cache creation/read behavior across dynamic tails
• bump @librechat/agents to 3.1.75

Tests

• npm test -- src/agents/tests/AgentContext.test.ts --runInBand
• RUN_ANTHROPIC_PROMPT_CACHE_LIVE_TESTS=1 RUN_BEDROCK_PROMPT_CACHE_LIVE_TESTS=1 npm test -- src/agents/tests/AgentContext.anthropic.live.test.ts src/agents/tests/AgentContext.bedrock.live.test.ts src/agents/tests/AgentContext.test.ts --runInBand
• npm run build
• npm pack --json

Optional live verification

• RUN_ANTHROPIC_PROMPT_CACHE_LIVE_TESTS=1 ANTHROPIC_API_KEY=... npm test -- AgentContext.anthropic.live.test.ts --runInBand
• RUN_BEDROCK_PROMPT_CACHE_LIVE_TESTS=1 BEDROCK_AWS_REGION=... BEDROCK_AWS_ACCESS_KEY_ID=... BEDROCK_AWS_SECRET_ACCESS_KEY=... npm test -- AgentContext.bedrock.live.test.ts --runInBand

[danny-avila]

@codex review

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Codex Review: Didn't find any major issues. Can't wait for the next one!

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[danny-avila]

@codex review

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Reviewed commit: 17ac9aec20

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[danny-avila]

@codex review

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Codex Review: Didn't find any major issues. Chef's kiss.

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[danny-avila]

@codex review

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Codex Review: Didn't find any major issues. Breezy!

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[danny-avila]

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Reviewed commit: 18525b2918

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[danny-avila]

@codex review

[danny-avila]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Chef's kiss.

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