cache: fix background stale-while-revalidate subrequest failure for HTTP/2 downstream sessions

[stillnovice]

Problem

When a proxy enables stale-while-revalidate, an HTTP/2 downstream request holding the cache write lock spawns a background subrequest that always fails (reported in #1033):

1. SubrequestSpawner::spawn_background_subrequest serializes the downstream request as HTTP/1 text via to_h1_raw(). For an HTTP/2 session, pseudo_raw_h1_request_header() wrote HTTP/2 as the request-line version, and httparse rejects it with Error::Version.
2. process_subrequest then hits the None => return branch, so cache_write_lock() is never taken — the write permit is dropped while still held. Debug builds panic with Dangling cache lock started!; release builds release the permit as dangling, the origin never revalidates, and every subsequent stale request on the key repeats the failure.

Fix

http_req_header_to_wire() now serializes Version::HTTP_2 as HTTP/1.1. This is safe: its only production callers are the HTTP/1 client writer (which only ever carries an HTTP/1.x version) and pseudo_raw_h1_request_header(), whose output is consumed exclusively by HTTP/1 parsers (the dummy SessionV1 subrequest session).

Tests

• test_h2_request_to_wire_uses_h1_version: unit regression test asserting an H2-versioned RequestHeader serializes with an HTTP/1.1 request line and parses via httparse.
• test_pseudo_raw_h1_request_header_is_valid_h1: end-to-end H2 test (real h2 handshake over a duplex stream) exercising the exact pseudo_raw_h1_request_header() path used by SubrequestSpawner.

Verified both tests fail with httparse::Error::Version when the fix is reverted, and pass with it; existing wire-format and pingora-proxy subrequest tests still pass; clippy clean.

Fixes #1033

[stillnovice]

Note on CI: the pingora (nightly) job fails at cargo fmt --all -- --check on pingora-cache/src/key.rs:187 — a doc-comment style change (*/ → */) introduced by a recent nightly rustfmt. That file is untouched by this PR; the failure is pre-existing style drift against the latest nightly toolchain, not caused by these changes. Stable (1.97.1) and MSRV (1.85.0) jobs pass. Happy to include the one-line rustfmt fix here if maintainers prefer.