fix(fastify): catch thrown route errors

[andrewtdiz]

Summary

Closes #928.

• Add Perry try/setjmp traps around native Fastify route handlers, onRequest/preHandler hooks, and custom setErrorHandler callbacks in both perry-ext-fastify and the in-tree stdlib Fastify implementation.
• Normalize sync throws and rejected Promises into Fastify-style error responses instead of letting js_throw exit the process when no try frame is active.
• Render built-in runtime Error values through js_error_get_message so default 500 bodies are stable and do not use generic JSON stringify on ErrorHeader.
• Add sync/async thrown-route regression coverage and assert the server remains alive after both failures.

Root Cause

Async flag propagation was not the primary issue: async arrows/functions already reject their returned Promise for top-level throws. The native Fastify bridge was missing the framework error boundary. It invoked raw Perry closures from Rust without an active js_try_push frame, so sync throws from handlers, hooks, or nested helpers reached js_throw with TRY_DEPTH == 0 and terminated the process.

Verification

• cargo fmt --check
• cargo check -p perry-stdlib -p perry-ext-fastify
• cargo build --release -p perry -p perry-runtime -p perry-stdlib -p perry-ext-fastify
• PERRY_BIN="$PWD/target/release/perry" scripts/run_fastify_tests.sh — fastify-tests: 10 passed, 0 failed
• cargo test --release -p perry-ext-fastify — 10 passed
• cargo test --release -p perry-stdlib fastify — 22 passed

Contributor Metadata

Per CONTRIBUTING.md, this PR does not bump [workspace.package], CLAUDE.md, or CHANGELOG.md; release metadata is left for the maintainer at merge time.