fix(links): decode base64url dialog audio in the whisper links

[howethomas]

The Groq and Hugging Face whisper links decoded inline dialog audio with plain base64.b64decode() whatever the dialog's encoding said. vCon core draft-04 allows only base64url, json or none for encoding, and producers are moving inline audio to unpadded base64url. On that input the old code either raised on the missing padding or, because b64decode does not validate by default, silently dropped - and _ and returned corrupted audio that was then transcribed.

Changes

• common/lib/dialog_body.py: decode_inline_dialog_body(dialog) -> bytes decodes by encoding.
  • base64url: padding restored, then a validating decode. base64.urlsafe_b64decode is not used because it also skips validation.
  • base64, or no encoding (vCons stored before the field was required): validating standard decode.
  • Line breaks and other whitespace are stripped first, so MIME-style bodies wrapped at 76 columns still decode, as they did before.
  • none, any other value, or invalid characters: ValueError, never a guess.
• Links: groq_whisper, hugging_face_whisper and wtf_transcribe use the helper. wtf_transcribe already branched on base64url but called urlsafe_b64decode without restoring padding, so it raised on unpadded bodies; fixed by the same change.
• Other b64decode sites were checked and left alone: SCITT key material and signing keys are not dialog bodies. common/vcon.py's write-time base64url validation does not restore padding either; noted, out of scope here.

Tests

• common/tests/lib/test_dialog_body.py: unpadded and padded base64url, legacy base64, missing encoding, bytes that encode to - and _ (asserting plain b64decode corrupts them and the helper does not), line-wrapped bodies, and every error path.
• Per-link tests for Groq, Hugging Face (new file; the existing ones are commented out pending API keys) and wtf_transcribe, including unpadded base64url.
• uv run pytest on the touched modules: 35 passed, 4 skipped (API-key and network gated), against 26 passed on main. The wider non-Docker suite: 684 passed, 20 skipped. The Redis-backed modules CI runs inside Docker were not run locally.

Refs CON-1112

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