🔒 Fix Cache Poisoning via Cache Key Collision

[undivisible]

🎯 What: This PR fixes a cache poisoning vulnerability caused by a cache key collision due to string interpolation that an attacker could easily bypass. The cache_key was previously constructed using format!("{}:{}:{}", self.embedded, query.mode().as_str(), query.to_query_string()), which meant an attacker could exploit strings with user input containing delimiters to cause a cache collision and overwrite results for a victim's search query.

⚠️ Risk: Cache poisoning where an attacker overrides caching by colliding values with specific delimiters, rendering the system to serve unauthorized, incorrect, or malicious cache results to users for standard searches.

🛡️ Solution: The solution avoids string interpolation and strictly uses the type system to enforce caching logic. We convert the SearchClient's cache key to a (bool, SearchMode, String) tuple, as TtlCache supports keys that implement Eq + Hash. We've added Hash to SearchMode to support this fix.

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PR created automatically by Jules for task 17952989123137737949 started by @undivisible

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Note

Low Risk
Localized cache-key change with no API or search behavior changes beyond fixing incorrect cache hits.

Overview
SearchClient no longer builds cache keys with format! and colon-separated fields. Keys are now (embedded, SearchMode, query_string) so embedded flag, mode, and serialized query are hashed as separate fields instead of one ambiguous string.

SearchMode derives Hash so it can participate in that tuple key for TtlCache.

^{Reviewed by Cursor Bugbot for commit 60c2c65. Configure here.}

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