fix(tools): enforce registry result contract

[teknium1]

Summary

The registry now accepts strings and the supported multimodal envelope, converting unsupported handler return types into structured JSON contract errors.

Changes

• Implement the behavior at the existing subsystem chokepoint rather than adding a parallel runtime.
• Preserve current prompt caching, message-role alternation, profile isolation, and provider wire compatibility.
• Add focused regression coverage for the corrected success, failure, interruption, or restart paths.

Validation

44 registry tests + 207 model-tools/computer-use tests passed.

tests/tools/test_registry.py | 64 ++++++++++++++++++++++++++++++++++++++++++++
 tools/registry.py            | 41 +++++++++++++++++++++++++---
 2 files changed, 102 insertions(+), 3 deletions(-)

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