Make H2 response header reads cancellation safe

[Pybsama]

Closes #934.

What this changes

Http2Session::read_response_header() now drives the existing
poll_read_response_header() state machine through std::future::poll_fn.
That poll path restores the pending ResponseFuture to the session before
yielding, so an internal read timeout or external future cancellation cannot
drop the only continuation while leaving the session alive.

The existing timeout error and H2 response-error mappings are unchanged.

Tests

Added in-memory HTTP/2 regressions that:

• time out through Http2Session::read_timeout, then resume and read a delayed
  200 response;
• cancel the async read through an external Tokio timeout, then resume and read
  the same delayed response.

Local validation:

• cargo test -p pingora-core --lib
• cargo test -p pingora-core --lib --bins --tests --no-fail-fast
• cargo test -p pingora-core --doc
• cargo check --workspace
• cargo fmt --all -- --check
• cargo clippy -p pingora-core --lib -- --allow=unknown-lints --deny=warnings

The full pingora-core library result was 481 passed, 1 ignored, 0 failed.