fix(runtime): align AbortSignal fallback

[NathanFlurry]

• Harden the native-first AbortController fallback with standard reasons, idempotence, and exact reason propagation.
• Validate AbortSignal.any() inputs and clean up listeners deterministically.

[NathanFlurry]

Stack for rivet-dev/agentos

• #1869 test(runtime): cover rich media across agent adapters
• #1868 fix(codex): reject unsupported prompt media
• #1867 fix(runtime): address polyfill review edge cases
• #1862 refactor(runtime): use upstream file URL helpers
• #1861 refactor(runtime): remove dead builtin wrappers
• #1866 fix(runtime): use native RGI emoji regexp support
• #1863 refactor(runtime): remove bootstrap compatibility stubs
• #1865 refactor(runtime): bootstrap final event and abort globals
• #1864 refactor(runtime): bootstrap final URL constructors
• #1860 refactor(runtime): use Undici FormData
• #1859 refactor(runtime): use upstream Blob and File
• #1856 fix(runtime): align AbortSignal fallback 👈
• #1855 fix(runtime): align DOM event fallback
• #1858 refactor(runtime): use upstream Web Streams
• #1857 refactor(runtime): use upstream text encoding
• #1853 refactor(runtime-browser): use upstream Node polyfills
• #1854 refactor(runtime): use upstream EventEmitter
• #1852 refactor(runtime): share stdlib modules across ESM and Commo …
• #1851 fix(runtime): use upstream WHATWG URL parser
• main

Get stack: forklift get 1856
Push local edits: forklift submit
Merge when ready: forklift merge 1856

_{change qxupvuop}