harness: surface provider error when schema output file is missing

[santoshkumarradha]

Problem

When Agent.harness(..., schema=...) is used and the provider fails before writing .agentfield_output.json (for example opencode run returning Session not found), the SDK currently returns a generic schema error:

Schema validation failed after parse and cosmetic repair attempts.

This hides the real provider failure and makes debugging look like a schema problem.

Reproduction

• Provider: opencode
• Schema mode enabled (schema=...)
• CWD: /tmp/dvga
• Provider exits with Session not found
• No .agentfield_output.json is created

Observed result:

• is_error=True
• parsed=None
• error_message="Schema validation failed after parse and cosmetic repair attempts."

Expected result:

• Preserve provider error (e.g. Session not found) and include missing output-file context.

Root Cause

HarnessRunner._handle_schema_output() always returns the generic schema-validation error when parse_and_validate() returns None, even if raw.is_error=True from the provider and the output file was never created.

Proposed Fix

In _handle_schema_output():

• If raw.is_error and schema parsing failed, return provider error details.
• If output file is missing, append context like:
  • Output file was not created at {path}

This keeps the schema path for normal validation failures while preserving root-cause visibility for provider/runtime failures.