fix(memory): render fields without init_value as empty on init - #4213

Description

initialize_memory_files only seeded template variables for fields that declared an init_value. Fields without one were omitted from extra_fields, so when MemoryFileUtils.write rendered the content_template (via render_template, which uses Jinja DebugUndefined), their {{ field }} placeholders were written verbatim into freshly initialized memory files such as identity.md.

The initialized file then enters the memory-extraction prompt as existing-memory context, and a smaller VLM can echo the placeholder text back, causing parse_error extraction failures.

Human Involvement

• A human participated in the implementation or review loop
• This PR was generated entirely by AI agents without human participation in the loop

Related Issue

Fixes #4207

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• In openviking/session/memory/memory_type_registry.py, initialize_memory_files now seeds every schema field into the template variables, defaulting a missing init_value to an empty string. Fields without an explicit init_value therefore render as empty instead of leaking the literal {{ field }} text.
• Added a regression test test_initialize_memory_files_renders_fields_without_init_value_as_empty in tests/unit/session/test_memory_policy.py.

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

The fix was validated with a focused harness that exercises the project's own TemplateUtils.render (the exact renderer path used by MemoryFileUtils.write). Before the change, {{ no_init }} / {{ also_no_init }} placeholders leaked into the rendered output; after the change, those fields render as empty with no placeholder leakage. The added unit test mirrors the existing test_initialize_memory_files_respects_memory_type_filter pattern and will run in CI.

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Screenshots (if applicable)

N/A

Additional Notes

This addresses the structural cause (option 1 from the issue): a missing init_value is now treated as an empty string during initialization rendering, which also covers any other memory type whose content_template references a field without an init_value — not just identity.