⚡ Bolt: Optimize string allocations in casing functions

[cloudesize67-cmd]

💡 What: Modified string generation in xdk-lib/src/casing.rs to avoid using .collect::<Vec<_>>().join("") in Casing::Pascal and avoided .collect::<String>() when concatenating characters in camel_case.

🎯 Why: In Casing::Pascal, calling .collect::<Vec<_>>().join("") allocated an unnecessary intermediate Vector just to join the strings, consuming unnecessary CPU cycles and memory. Collecting directly to a String using .collect::<String>() avoids this intermediate allocation. In camel_case, collecting a character into a String via to_lowercase().collect::<String>() is not faster, we now simply collect the lowercased character iterator directly into a String using .collect::<String>() which uses size_hint to pre-allocate capacity.

📊 Impact: Reduces allocations during parsing and casing conversions, which can occur thousands of times when evaluating large OpenAPI specs, leading to improved memory efficiency and marginally faster SDK generation time.

🔬 Measurement: Verify tests run smoothly using make check and make test-generator. Tests passed successfully indicating no functional change.

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PR created automatically by Jules for task 15234192539336072315 started by @cloudesize67-cmd

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