Use shared field guards for TransformConfig numeric settings

[HarshRajSinghania]

Summary

Route TransformConfig numeric validation through the shared helpers in _field_guards instead of hand-rolling type and range checks for crf, gop_seconds, and chunk_size_bytes.

Motivation

Fixes #510. The issue asked to convert the three remaining guards and to stop matching only the field name in the refusal tests, so a type refusal and a range refusal stay distinguishable.

Implementation

• crf uses require_int_in_range(..., minimum=0, maximum=51).
• gop_seconds stays optional; when set it uses require_positive_float.
• chunk_size_bytes stays optional; when set it uses require_positive_int.
• isinstance(..., bool) is gone from transform.py.
• Valid configurations are unchanged: None remains legal for the two optional fields, and accepted numeric values are the same as before.

Refusal messages now follow the shared helpers (for example crf must be in [0, 51], got 52 rather than crf must be between 0 and 51). That is the existing helper wording, not a new contract.

Testing

The ten existing invalid cases now each assert their exact helper message. The valid-settings case still covers crf=0, gop_seconds=1, and chunk_size_bytes=1. Defaults (gop_seconds=None, chunk_size_bytes=None) remain accepted by the existing TransformConfig() construction in test_pipeline_version_is_a_content_hash.

The full project suite needs uv, ffmpeg, and a C toolchain, which were not available in this environment. The change is a direct substitution onto already-tested helpers plus tightened assertions around those helpers.

[github-actions]

👋 Hi @HarshRajSinghania — thanks for the contribution! To keep starter issues available
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