feat: add JIT warmup phase and measurement to Java instrumentation

[HeshamHM28]

Summary

• Adds a dedicated untimed warmup loop before the timed measurement loop in Java test instrumentation
• Previously, all inner loop iterations were timed from the start, meaning early iterations ran on interpreted/unoptimized JVM bytecode and skewed benchmark results
• The warmup phase executes the target method 50 times (configurable via CODEFLASH_WARMUP_ITERATIONS env var) before any timing begins, giving the JIT compiler a chance to optimize the hot path
• Warmup exceptions are silently caught so failures during warmup don't break the benchmark run