fix(extensions): align verifiability test vectors with the reference implementation

[xibz]

Follow-up to #1386. Some of the verifiability extension's published test vectors didn't match the reference implementation, and some couldn't be reproduced from the event JSON printed next to them, so implementations couldn't pass them as written.

Proposed Changes

• Cases 1 and 2 carried an outdated payload: a bare digest instead of the JSON material body.
• Cases 6a and 6b were corrupted, and their signatures didn't verify against the test key.
• Cases 1–5 printed their events with different indentation from the events that were signed. Whitespace inside data is part of the signed bytes, so the digests didn't match the events shown.
• Every vector is now the reference implementation's expected value, and Cases 1–5 print their events exactly as its test fixtures, so signing each printed event reproduces its vector.
• The HTTP examples are consistent too: the structured-mode examples use the Case 1 event so the signed one carries Case 1's material, the binary-mode example is signed over its body as shown, and each Content-Length matches its body.

No normative text changes. Only the example values are updated.

Release Note

Verifiability extension: corrected the test vectors and HTTP examples so every published value reproduces from the event shown.