fix(processor): redact code execution errors

[XnLemon]

Objective

Close one Phase2 redaction gap where code execution errors are emitted as model-visible post-processing messages.

Changes

• Redact ExecuteCode errors before emitting Code execution failed result events.
• Extend the shared platform redactor with generic whole-line Authorization and Cookie fallbacks.
• Add platform redactor regression coverage for non-standard Authorization schemes and multi-value Cookie headers.
• Add code execution regression coverage for Authorization, api_key, URL userinfo, and Cookie secrets in executor errors.

Validation

• go test ./platform -run Redactor
• go test ./internal/flow/processor -run CodeExecutionResponseProcessor
• go vet ./platform ./internal/flow/processor
• git diff --check
• open-code-review subagent loop: final review 0 High / 0 Medium / 0 Low

Known Risks / Limitations

• Scope is intentionally limited to shared redaction patterns and the code-execution error event path.
• Full go test ./internal/flow/processor still has existing Windows path-separator failures in skill path tests, unrelated to this change.

Follow-up

• Continue Phase2 closure sweep across remaining model-visible error and governance output surfaces in the next stacked increment.