fix: replace peek() with SHA-256 hash to prevent cache poisoning (AC-2)

[3em0]

Summary

• Security fix: BufferedReader.peek() only returns the first ~8192 bytes (buffer size) of a file. Cache keys generated from peek() output are vulnerable to collision — two files sharing the same header but different content produce identical cache keys, enabling cache poisoning and information disclosure.
• Replace peek() with streaming SHA-256 hash of full file content in three affected functions: get_file_bytes(), get_input_str(), and get_image_question()
• File pointer is reset via seek(0) after hashing so downstream LLM calls still read the complete file
• Also fixes a resource leak in get_image_question() (open() without close)

Affected Functions

Function	Used By	Old Behavior	New Behavior
get_file_bytes()	OpenAI audio adapter	file.peek() → ~8KB	sha256(file.read()) → full content hash
get_input_str()	Replicate adapter	str(image.peek()) + question	sha256(image) + question
get_image_question()	MiniGPT4 adapter	str(open(img).peek()) + question	sha256(img) + question

Attack Scenario

1. Attacker sends img_A (legitimate) + question → answer cached with key derived from first 8KB
2. Attacker constructs img_B with identical first 8KB but completely different content
3. Attacker sends img_B + same question → cache returns img_A's answer (LLM never called)

See tests/security/AC-2_cache_poisoning_via_peek.md for the full vulnerability report with CVSS scoring.

Test plan

• 17/17 unit tests passing (tests/unit_tests/processor/test_pre.py)
• New tests verify: no collision between files with same header, identical files still match, file pointer reset after hashing, file path input works
• Verified fix against PoC scripts (tests/poc_ac2_peek_collision.py, tests/poc_ac2_e2e_poisoning.py)
• Reviewers: verify existing adapter tests still pass with the new hash-based keys

🤖 Generated with Claude Code

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[sre-ci-robot]

Welcome @3em0! It looks like this is your first PR to zilliztech/GPTCache 🎉

[3em0]

@xiaofan-luan hi！ how it is going?

[3em0]

/assign @cxie

Hi @cxie and @xiaofan-luan, DCO is fixed and the PR is ready for review.
When you have a moment, could you please take a look? If it looks good, an /approve would help move it forward. Thanks!