ENT-969: Add TestPatternStreamProducer as a built-in video source type

[NVergunst-ROBO]

What does this PR do?

Adds a synthetic 1920x1080 test pattern with colored grid and live clock

Type of Change

• New feature (non-breaking change that adds functionality)

Testing

• I have tested this change locally

also tested with full changes in roboflow app and inference for manufacturing on a local jetson device.

Test details:
test_is_test_pattern_reference_accepts_valid_and_rejects_invalid -- Verifies the guard function correctly identifies "TestPatternStreamProducer" strings (with whitespace/suffixes) and rejects integers, RTSP URLs, partial matches, and empty strings.
test_generate_frame_produces_valid_image -- Confirms the frame generator outputs a 1920x1080 BGR uint8 image that isn't all black.
test_discover_source_properties_for_test_pattern -- Checks the producer reports correct width, height, fps, is_file=False, and is_reconnectable=True.
test_video_source_starts_and_reads_frames_from_test_pattern -- End-to-end: passes "TestPatternStreamProducer" as a video reference to VideoSource, starts the stream, reads a frame, and verifies the source is RUNNING with stream-mode buffer strategies.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in hard-to-understand areas
• My changes generate no new warnings or errors
• I have updated the documentation accordingly (if applicable)

Additional Context

[dkosowski87]

Do we plan to check it manually or through some automatic tests? I'm wondering whether it wouldn't be better to move the test_pattern_producer to the test directory, so it doesn't pollute the core inference scope. WDYT?

[PawelPeczek-Roboflow]

Probably test_pattern_producer is intended name