🧪 test: add comprehensive unit test suite for image_utils module

[chottokun]

Add comprehensive unit test coverage for src/app/image_utils.py in src/tests/test_image_utils.py.

🎯 What

Addresses the missing test file for src/app/image_utils.py.

📊 Coverage

• SSRF Defense Functions: resolve_safe_url_async and is_safe_url_async across valid public IPs, private/loopback/link-local IP addresses, invalid schemes/hostnames, and socket resolution errors (gaierror, ValueError, TypeError).
• Safe Network Backend: SafeNetworkBackend connection parameter overrides (connect_tcp, connect_unix_socket, and sleep).
• Image Conversion Helper: _decode_and_convert_image with valid image bytes and corrupted byte streams.
• Base64 Decoding: Success cases, payloads exceeding 15MB MAX_FILE_SIZE, and corrupted Base64 strings.
• URL Download & Redirects: Happy path downloads, SSRF blocked URLs, file size limit enforcement during chunked streaming, valid safe redirects, missing Location header handling, and max redirect limits (MAX_REDIRECTS).
• Configuration Limits: Verification of MAX_IMAGE_PIXELS (20M pixels) decompression bomb protection.

✨ Result

Added 29 unit test cases achieving high test coverage for image_utils.py and ensuring no regressions across existing multimodal test suites.

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PR created automatically by Jules for task 571073663461290150 started by @chottokun

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