fix(tap): recognize signed empty HTTP requests

[nkavian]

Summary

• Recognize signed empty content in the TAP HTTP example when Content-Digest is present without framing headers.
• Exercise acceptance, invalid-digest rejection, and replay rejection over real loopback HTTP.
• Pin the shared contract containing original-query spelling regression cases.

Verification

• make sync and make verify passed on Python 3.11, 3.12, 3.13, and 3.14, including coverage and isolated package consumers.
• All supported runtime, MPP, x402, and TAP shared conformance cases passed.
• The empty-request regression failed before the fix and passes afterward.

Depends on inflowpayai/inflow-specs#26. Merge that fixture update first. No payment, key-trust, signature-lifetime, or replay-policy changes.