Add regression tests for seven cache and HTTP behaviours

[Trenyx]

Refs #1035

@andrewhavck: these are the tests from our email thread.

Tests only: seven unit tests (+123 lines) in the existing test modules. No change to library code, no new dependencies.

Test	What it pins down
filters::tests::test_upstream_request_filter_revalidates_with_if_modified_since	Revalidation sends Last-Modified as If-Modified-Since, so the origin can answer 304
variance::test::test_value_differentiates_key	The header value is part of the variance key: gzip and br don't share a variant
meta::tests::test_cache_meta_serve_stale_window_end_is_inclusive	Each serve-stale window includes its last second, as is_fresh includes fresh_until
tests::an_uncacheable_response_makes_waiting_readers_give_up	An uncacheable response gives the cache lock up, so waiting readers all proceed uncached
authority::tests::classify_raw_target_authority_scheme_case_insensitive	HTTP:// gets the same ambiguous-authority rules as http:// (RFC 3986 schemes are case-insensitive)
tests::test_remove_header_drops_case_name	Removing a header also drops its case-preserved name
tests::test_ser_de_keeps_repeated_headers	Repeated fields such as Set-Cookie survive a serialize/deserialize round trip

How they were checked

On a copy at 4487f7b (current main):

• With the tests added to unchanged code, cargo fmt --all -- --check, cargo clippy --all-targets -p pingora-cache -p pingora-http -p pingora-header-serde -- --allow=unknown-lints --deny=warnings and cargo test --lib for those three crates all pass (193, 58 and 10 tests, including the new ones).
• With each of the seven bugs from the issue applied one at a time, exactly its matching test fails and every other test still passes.

These tests were written with Claude Code during a planted-bug run by Trenyx.