Encode ObjectKey len as -1 to support 1024 char keys

[shikhar]

Fixes #46, #39

[greptile-apps]

Greptile Summary

Changed binary encoding of ObjectKey length from storing the actual value to storing length - 1, enabling support for the full 1024-character keys that are already allowed by validation in src/types.rs. The 10-bit field previously supported 0-1023, but now supports 1-1024 by encoding the value minus one (similar to how kind_len was already encoded).

Key changes:

• Bumped cache version from 2 to 3 (incompatible format change)
• Added validation to reject zero-length keys in CacheKeyHeader::new
• Changed boundary check from key_len >= 1024 to key_len > 1024
• Updated encoding to store key_len - 1 in the 10-bit field
• Updated decoding to add 1 when extracting the key length
• Updated all tests to reflect new maximum (1024) and validation requirements

This fix resolves the mismatch between validation (allowing 1024-char keys) and encoding (previously rejecting them), eliminating the 500 errors that occurred when attempting to cache objects with exactly 1024-character keys.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The change is well-tested with comprehensive unit tests and property-based tests covering the new 1024-character limit. The version bump ensures backward incompatibility is handled explicitly, preventing silent data corruption. The implementation is symmetric (encoding/decoding both apply the minus-one transformation), and the boundary validations are correct.
• No files require special attention

Important Files Changed

Filename	Overview
src/cache.rs	Encodes key length as length - 1 to support full 1024-char keys; version bumped to 3

Sequence Diagram

sequenceDiagram
    participant Client
    participant CacheKey
    participant CacheKeyHeader
    participant Storage

    Note over Client,Storage: Encoding Process with Version 3
    Client->>CacheKey: encode with kind, object key, page id
    CacheKey->>CacheKeyHeader: create header with lengths
    
    alt Invalid key length
        CacheKeyHeader-->>CacheKey: Return error
    else Valid key length (1 to 1024 chars)
        Note over CacheKeyHeader: Encode length as (length minus one)
        CacheKeyHeader->>CacheKeyHeader: Pack into 5 byte header
        CacheKeyHeader-->>CacheKey: Return header bytes
        CacheKey->>Storage: Write header, kind, key
    end

    Note over Client,Storage: Decoding Process with Version 3
    Storage->>CacheKey: decode from storage
    CacheKey->>CacheKeyHeader: parse header bytes
    CacheKeyHeader->>CacheKeyHeader: Extract encoded length
    Note over CacheKeyHeader: Restore actual length by adding one
    CacheKeyHeader-->>CacheKey: Return actual key length
    CacheKey->>Storage: Read key bytes using length
    CacheKey-->>Client: Return decoded cache key

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