Add tests for interceptor and HTTP content feature interactions

[m-szymon]

Current tests cover request compression and header optimization independently, but do not cover the combined path or the key ordering/interaction cases.

Include:

• combined per-request overrides for request_compression and optimize_headers
• HTTP content features with load balancing / retries
• compression before signing
• URI rewrite before transmit

Expected outcome:

• overrides apply only to the customized request
• compression and header filtering still work with routing enabled
• signing uses the final compressed request
• transmitted URI matches the node selected by the query plan

Part of #41.