fix: harden Vary caching and collapsed forwarding

[sendya]

Summary

• Introduce deterministic request-header fingerprints and a collision-resistant v2 format for Vary identities.
• Harden Vary cache lifecycle handling, including wildcard, ignored, changed, removed, legacy, and concurrent-variant cases.
• Restore effective object-level collapsed forwarding without sharing responses across incompatible representations.
• Keep method, range, conditional, authentication, cache-control, and upstream-routing inputs isolated where Vary cannot express the distinction.

Motivation

Collapsed forwarding regressed after the flight key began fingerprinting the complete request header map. Request-local metadata such as X-Request-Idx, request IDs, and tracing fields then produced different keys for otherwise equivalent cache misses, causing every concurrent request to reach the origin.

Simply returning to an object-only key would make cold misses unsafe for different methods, ranges, authentication contexts, or response representations. Vary identity serialization also needed stronger field-boundary guarantees and compatibility with existing cache entries.

Implementation

• Serialize Vary selections using an explicit v2 schema that distinguishes field names, boundaries, absent fields, empty values, and value ordering.
• Retain fallback lookup support for legacy Vary cache keys.
• Select representation-specific object IDs before collapsing known Vary misses.
• Build the initial object-flight key from the object ID, HTTP method, and request controls that affect response semantics outside Vary.
• Inspect the origin response's actual Vary fields before fan-out.
• Re-collapse incompatible waiters under representation-specific keys instead of returning the leader's response.
• Merge concurrent Vary root metadata updates so newly discovered variants are not lost.
• Add focused regression coverage for metadata-only differences, Vary representation isolation, request-control isolation, wildcard handling, legacy compatibility, and concurrent variant updates.

Impact

Equivalent concurrent full-cache misses once again collapse to a single origin request, including requests carrying unique correlation metadata. Requests that select different representations or carry response-altering controls remain isolated, preserving response correctness and cache safety.

Validation

• go test -count=1 ./server/middleware/caching ./pkg/x/http ./proxy
• CGO_ENABLED=1 go test -race -count=1 -run '^TestObjectFlight' ./server/middleware/caching
• go vet ./server/middleware/caching ./pkg/x/http ./proxy
• go test -count=1 -v ./tests/all-features/caching -run '^TestCollapsedForwardingObjectFlight$' against a local Tavern instance
• Static Tavern build
• git diff --check