fix(media): validate PNG headers

[Promise Emmanuel Oluwadare (promiseeuler)]

Summary

While using viam-sdk==0.80.0, I found that ViamImage accepts PNG data with an invalid signature or a non-IHDR first chunk and reports dimensions from those bytes. For example, an invalid payload can be reported as 640x480.

The PNG parser was constructing ValueError objects without raising them, and it compared a bytes signature with a list of integers. This change raises the validation errors and compares the signature with bytes, allowing ViamImage to fall back to unknown dimensions for malformed PNG data.

Regression coverage includes both an invalid signature with a plausible IHDR chunk and a valid signature with an invalid first chunk.

Validation

• uv run ruff format --check src/viam/media/video.py tests/test_media.py
• uv run ruff check src/viam/media/video.py tests/test_media.py
• uv run pyright — 0 errors (1 existing warning in src/viam/module/types.py)
• uv run pytest -q tests/test_media.py — 9 passed
• uv run pytest -q — 866 passed, 5 skipped because viam-server is not installed
• git diff --check

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[Naveed Jooma (njooma)]

nice find