perf(runtime): eliminate hot-path string allocations

[arsenz]

What kind of change does this PR introduce?

Performance optimization (perf)

What is the current behavior?

Currently, the Edge Runtime performs unnecessary heap allocations and UTF-8 validations on every incoming request in the hot-path:

1. UserWorkerResponse allocates a new String for status_text on every response, even though the underlying hyper status reasons are static.
2. Header parsing unconditionally performs .to_str() UTF-8 validation before mapping to Deno ByteString.
3. The server abort logging loop redundantly calls req_uri.to_string().

These allocations cause memory allocator lock contention and CPU overhead under high concurrency.

Please link any relevant issues here.
Resolves #712

What is the new behavior?

This PR refactors these hot-paths to use zero-copy mechanisms to bypass the allocator:

• Changed status_text in UserWorkerResponse from String to &'static str to map canonical hyper reasons directly to memory without allocation.
• Refactored header mapping to map raw bytes directly to Deno ByteString, bypassing .to_str() UTF-8 validation overhead.
• Replaced .to_string() allocation on request URIs during abort logging with dynamic Display formatting.

Additional context

Baseline load testing using k6/specs/simple.ts shows significant improvements after these changes:

• Throughput: Increased from ~11,134 RPS to ~12,872 RPS (+15.6%)
• Latency: Average request duration dropped from 1.06ms to 915µs (breaking the sub-millisecond barrier).
• Consistency: p95 latency improved from 1.30ms to 1.15ms.

These changes are purely internal optimizations and do not alter any external APIs or behavior.