test: prove explicit workflow proposal recovery

[enricopiovesan]

Summary

Adds end-to-end evidence that rejected workflow proposals do not retry implicitly: only a corrected, explicitly re-submitted proposal can execute.

Governing Spec

• 109-runtime-workflow-proposals

Project Item

Closes #1166.

Definition of Done

• Rejection remains a structured denial with a stable code.
• Recovery uses a separately corrected proposal and explicit execution.
• The successful recovery retains the existing governed trace assertions.

Validation

• cargo test -p traverse-mcp --test proposal_tests corrected_reproposal_requires_a_new_explicit_execution_after_rejection -- --exact
• cargo fmt --all --check
• git diff --check