test(signing): re-pin the vendored vectors at AdCP 3.1.8

Review follow-up. main moved to 3.1.8 (#992) while this branch pinned 3.1.1,
so test_manifest_pins_the_spec_version_the_sdk_targets failed once the two met.

Only spec_version and source change: the 3.1.1 and 3.1.8
dist/compliance/.../test-vectors/request-signing trees are byte-identical, and
regenerating the manifest reproduces all 43 file hashes unchanged. That is the
pin doing its job -- it forced an explicit re-vendor decision on a spec bump
rather than letting the old set ride silently.

Note for the next bump: `python -m tests.conformance.signing.vectors --write`
regenerates `files` only and carries spec_version/source forward, so those two
fields need editing before the regen, not by it.

Refs #975.

Author: KonstantinMirin

tests/conformance/signing/vector_manifest.json

@@ -1,7 +1,7 @@
 {
   "$comment": "Completeness pin for the vendored AdCP request-signing conformance vectors. Generated by `python -m tests.conformance.signing.vectors --write`; do not hand-edit. Graded by tests/conformance/signing/test_vector_completeness.py.",
-  "spec_version": "3.1.1",
-  "source": "adcontextprotocol/adcp @ v3.1.1 -- dist/compliance/3.1.1/test-vectors/request-signing/",
+  "spec_version": "3.1.8",
+  "source": "adcontextprotocol/adcp @ v3.1.8 -- dist/compliance/3.1.8/test-vectors/request-signing/",
   "files": {
     "README.md": "74977dcb44eef69aee2b45e78e9fc111625e101d7e8646ed9f44b521aee396e8",
     "canonicalization.json": "83a15584bebb0ff006ffd99a4080f5a922ea56e9e590d516ee87becaaf15ad1b",