Bug: public API contact_sheet=True fails without an explicit config

[Ahmed-Hindy]

Summary

RenderKit.convert_exr_sequence_to_mp4(..., contact_sheet=True) exposes contact_sheet_config as optional, but calling the public API with contact-sheet mode enabled and no config immediately raises ConfigurationError. The CLI builds a default contact-sheet config, while the API leaves users to discover the builder invariant at runtime.

Current behavior

The public API signature is:

def convert_exr_sequence_to_mp4(..., contact_sheet: bool = False, contact_sheet_config: Optional[ContactSheetConfig] = None, ...)

But the implementation only does:

if contact_sheet:
    config.with_contact_sheet(True, contact_sheet_config)

ConversionConfig.__post_init__() then rejects contact sheet mode without a config.

Reproduction

Run this against current origin/main (a582621):

from renderkit.api.processor import RenderKit

rk = object.__new__(RenderKit)
try:
    rk.convert_exr_sequence_to_mp4("render.%04d.exr", "out.mp4", fps=24, contact_sheet=True)
    print("returned")
except Exception as exc:
    print(type(exc).__name__, str(exc))

Observed:

ConfigurationError Contact sheet config is required when contact sheet mode is enabled

Expected behavior

Either:

• convert_exr_sequence_to_mp4(contact_sheet=True) should create a default ContactSheetConfig, matching CLI behavior; or
• the API should require contact_sheet_config when contact_sheet=True and document/validate that explicitly before building the config.

Impact

Python API users get a surprising runtime failure from an argument that appears optional. This is separate from the existing contact-sheet CLI crash because it affects the public RenderKit convenience API and can be reproduced without invoking the broken CLI command.

Likely fix areas

• src/renderkit/api/processor.py convert_exr_sequence_to_mp4()
• src/renderkit/core/config.py contact-sheet config validation
• Add API regression tests for contact_sheet=True with and without an explicit config.