conformance: deconflict expected.error_code field semantics

[jithinraj]

Problem

The expected.error_code field in workflow conformance fixtures (and other conformance suites) carries two distinct meanings:

1. Validation error code (normative) - The code a validator MUST emit when rejecting invalid input (e.g., E_WORKFLOW_SELF_PARENT)
2. Runtime error code (informational) - The code that might appear at runtime for edge cases that pass structural validation

This overload makes it ambiguous whether a test harness should assert on error_code for edge-case fixtures where expected.valid is true but an error_code is present for documentation.

Currently the JSON Schema enforces error_code is required when valid: false (via allOf/if/then), which is correct. But the field name itself suggests it's always a validation output, which could confuse cross-language implementors.

Proposal

Split into two distinct fields with a deprecation window:

Phase 1: Add new fields (next conformance version bump)

• validation_error_code - Normative. Required when valid: false. Test harnesses MUST assert this.
• meta.runtime_error_code - Informational. Optional. Documents what code might appear at runtime. Test harnesses MUST NOT assert this.

Phase 2: Deprecation (1 release cycle)

• Accept both error_code (legacy) and validation_error_code (new) for 1 release
• Fixture schema validates either field is present when valid: false

Phase 3: Remove legacy field

• Remove error_code from fixture schema
• All conformance suites use validation_error_code

Affected Fixture Suites

• specs/conformance/fixtures/workflow/ (valid.json, invalid.json, edge-cases.json)
• specs/conformance/fixtures/agent-identity/
• specs/conformance/fixtures/attribution/
• specs/conformance/fixtures/dispute/
• specs/conformance/fixtures/bundle/
• specs/conformance/fixtures/purpose/

Acceptance Criteria

• New field names documented in fixture README
• JSON Schema updated with oneOf for transition period
• All test harnesses (TS, Go) updated to read new field
• Legacy error_code removed after 1 release cycle
• Cross-language test harnesses updated