refactor(server): use tower-http DefaultPredicate.and() for compression skip rules

[momics]

Summary

crates/iroh-http-core/src/server.rs#L863-906 implements the compression-skip predicates as hand-written closures. tower_http::compression::predicate::DefaultPredicate already covers the standard set and chains via .and(). Use it instead, then add only the project-specific predicates on top.

Follow-up to the post-rework review at reviews/2026-04-30-post-rework-review.md §5.4.

Evidence

• tower_http::compression::predicate provides DefaultPredicate (size + content-type + already-encoded) and NotForContentType, SizeAbove building blocks.
• Our predicate logic (server.rs#L863-906) duplicates the "already encoded" and "no-transform" checks.

Impact

• Drift risk if upstream improves the default predicate (e.g. adds a media-type).
• More code to read and test for the same effect.

Remediation

1. Replace the manual closure stack with DefaultPredicate::default().and(NotForContentType::new("application/zstd")).and(NoTransformCacheControl) (where NoTransformCacheControl is a small custom predicate if not in tower-http).
2. Verify the threshold passes through (SizeAbove(min_body_bytes) is part of DefaultPredicate configurable via .with_size_above).
3. Remove the bespoke closures.

Acceptance criteria

• Predicate composition uses DefaultPredicate + named building blocks.
• All compression behaviour preserved (covered by existing tests).
• ~30 lines removed from server.rs.