[Runtime] Respect Accept header quality values when detecting JSON query requests

[cssbruno]

Priority

Medium — HTTP content negotiation correctness for generated query routes.

Context

runtime/app/backend.go routes contract query requests when either X-GOWDK-Query is truthy or the request Accept header appears to accept JSON.

The helper currently detects JSON with:

for _, part := range strings.Split(header, ",") {
    mediaType := strings.ToLower(strings.TrimSpace(strings.Split(part, ";")[0]))
    if mediaType == "application/json" || strings.HasSuffix(mediaType, "+json") {
        return true
    }
}

Problem

The parser ignores q quality values and other structured Accept-header semantics.

Examples:

Accept: application/json;q=0, text/html

This explicitly says JSON is unacceptable, but the current helper still returns true because it only checks the media type before ;.

Consequences:

1. A request can be routed as a query request even when JSON has q=0.
2. Browser/tool content negotiation can behave unexpectedly.
3. Future response negotiation is harder because the current helper is not a real Accept parser.
4. Tests may encode an overly permissive contract.

Proposed direction

Use a small structured Accept parser for generated-runtime routing decisions.

The helper should at minimum:

• split media ranges safely;
• parse parameters;
• treat missing q as 1;
• treat invalid q values consistently;
• ignore JSON media ranges with q=0;
• support application/json and application/*+json.

Acceptance criteria

• Accept: application/json still marks a query request.
• Accept: application/problem+json or another +json media type still marks a query request.
• Accept: application/json;q=0, text/html does not mark a query request unless X-GOWDK-Query is set.
• Invalid Accept headers have deterministic behavior documented in tests.
• Tests cover multiple header values, whitespace, uppercase media types, q-values, and wildcard behavior if supported.

Related

• Consolidate generated endpoint metadata into one canonical endpoint plan #674 — consolidate generated endpoint metadata into one canonical endpoint plan
• [Compatibility] Version and publish schemas for all public JSON contracts #689 — version and publish public JSON contracts