[Repo Assist] refactor(proxy): extract writeJSONResponse and forwardAndReadBody helpers

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Summary

Addresses duplicate code patterns identified in #2301 and #2302 by extracting two helpers in internal/proxy/handler.go.

Changes

writeJSONResponse(w, statusCode, body)

A package-level helper that centralises the three-line Content-Type + WriteHeader + json.NewEncoder(w).Encode(body) pattern. This pattern appeared identically in 4 places:

• Health check response
• Unknown GraphQL operation (403)
• DIFC write-blocked response (403)
• Strict-mode collection-filtered response (403)

forwardAndReadBody(ctx, w, method, path, body, contentType, clientAuth)

A method on proxyHandler that centralises the forward → read → error-handle pattern. This appeared in 3 places:

• GraphQL introspection passthrough
• Phase 3 forward in handleWithDIFC
• passthrough handler

Bonus: the original introspection path silently discarded io.ReadAll errors (respBody, _ := io.ReadAll(...)). The helper now logs and returns a 502 on failure — matching the behaviour of the other two sites and closing a silent failure mode.

Rationale

• Single place to adjust error handling, status codes, or logging for forwarding and JSON responses.
• Reduces handleWithDIFC and passthrough length; each call site is now 2–3 lines instead of 8–10.
• No functional changes to existing behaviour (except the introspection error-handling fix above).

Test Status

• gofmt -l . reports no formatting issues
• Build requires Go 1.25 which cannot be downloaded in the sandboxed environment (firewall blocks proxy.golang.org). The code was verified syntactically via gofmt -e and logic-reviewed manually.
• All existing tests are in proxy_test.go and proxy_coverage_test.go and test MatchRoute, MatchGraphQL, writeEmptyResponse, and copyResponseHeaders — none of which were modified.

Closes #2301
Closes #2302

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