perf: make AbortController opt-in per route

[mcollina]

Summary

• Make AbortController support opt-in per route with { abortController: true }
• Keep default fetch routes on the pre-AbortController fast path
• Preserve abort behavior for opt-in routes via route-local onRequestAbort and lifecycle cleanup
• Update docs, runtime tests, and TypeScript overloads for the opt-in API

Performance

Fastify.inject() GET benchmark, Node v24.15.0, 200k requests/run after warmup:

Variant	Avg req/s	Delta
before ee791c3	~73.9k	baseline
ee791c3	~45.1k	~39% slower
this PR default route	~74.2k	back to baseline
this PR { abortController: true }	~61.6k	~37% faster than ee791c3

Tests

• npm run lint
• npm test

[Tony133]

lgtm