fetch: accept HTTP/1.1 responses with more than 256 header fields - #34923

The `SAFETY` comment at [src/http/lib.rs:3801-3802](https://github.com/oven-sh/bun/blob/6e2b024886d9f89177e0100e16d7e68f4b5d03c3/src/http/lib.rs#L3801-L3802) for `unsafe { response.detach_lifetime() }` still says `response` borrows only `SHARED_RESPONSE_HEADERS_BUF / response_message_buffer`. On the new overflow path, `response.headers.list` instead borrows `SHARED_RESPONSE_HEADERS_OVERFLOW`'s heap buffer — whose element storage moves on the next `resize()`, so the "outlive this fn" phrasing no 

The overflow Vec is sized to `count_char(to_read!(), b'\n')`, but `to_read!()` at this point is the whole receive buffer — including any body bytes that arrived in the same read — so a >256-field response followed by a newline-dense body in the same packet(s) can size the Vec far beyond the actual header count (each `picohttp::Header` slot is 32 bytes, and a ~1 MB buffer of mostly-`\n` bytes maps to ~10-16 MB of slots). `SHARED_RESPONSE_HEADERS_OVERFLOW` is a process-lifetime static that is only