Validate complete upstream HTTP representation headers #307

🤖 Generated by the Agentic Engineer

Part of #283.

The source connector and contract probe inspect only the first repeated representation-header value. Contradictory content coding or media-type declarations can be admitted.

For publishers, operators and product consumers, the expected result is consistent validation and honest public metadata. The hypothesis is that enforcing the existing boundary at its real caller prevents this failure without adding steps to the valid workflow.

Acceptance criteria:

• Validate all received content-coding and content-type values before accepting response bytes. Reject ambiguous or unsupported representations while retaining absent coding, identity coding, and valid single JSON media types. Exercise real TLS responses.
• Prove the regression fails on current main and passes with the smallest root-cause repair.
• Keep affected documentation consistent and deliver through reviewed, passing CI and an actual merge.

Success signal: the focused regression and valid-path controls pass within this delivery round. Size: small to medium.