fix: clean up invalid materialized images

[kilisamemarisaaa]

Root cause

PreProcessStage detached every resolved image path from event cleanup. When a data URI or remote reference materialized invalid bytes, the path was treated as a durable attachment even though image processing had failed, leaving a media_image_* file behind.

Reproduction

On Windows with Python 3.12, passing an invalid data:image/png;base64 reference through PreProcessStage and then running event cleanup left the materialized file on disk. The regression occurred for both ordinary and Reply-chain images.

Fix

• Track newly materialized image files before validation.
• Validate only newly materialized files with the existing MIME detector.
• Keep invalid materialized files owned by the event so normal cleanup removes them.
• Preserve existing local/file URI behavior and valid PNG, GIF, JPEG, WebP, and BMP paths.
• Share ownership logic between ordinary and Reply-chain images.

Verification

• uv run pytest -q: 2968 passed, 1 skipped
• uv run pytest -q tests/test_preprocess_stage.py tests/test_media_utils.py tests/test_mattermost_adapter.py: 89 passed
• uv run ruff check .: passed
• uv run ruff format --check .: 505 files already formatted
• Added regression coverage for invalid materialized images in ordinary and Reply-chain messages.

Related: #10042

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Approved.

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