Fix Metashape tangential distortion import

[tianrking]

Summary

• Convert Metashape's tangential distortion coefficient names to Nerfstudio's OpenCV convention by swapping p1 and p2 during XML import.
• Add an end-to-end XML-to-transforms.json regression test with distinct coefficients.

Closes #3714

Why

Metashape assigns P1 to the (r^2 + 2x^2) term and P2 to the 2xy term, while OpenCV assigns those terms to p2 and p1, respectively. Copying the names directly therefore changes the imported camera distortion.

References: Agisoft Metashape 2.3 manual, Camera models and OpenCV camera calibration.

Validation

• Confirmed the new regression fails on main with 0.001 != -0.002, then passes with this change.
• Target regression plus tests/process_data/test_misc.py: 3 passed.
• Pre-commit trailing-whitespace, end-of-file, Ruff lint, and Ruff format hooks passed for both changed files.
• License-header check passed.
• Targeted Pyright: 0 errors (one local missing-open3d environment warning).

The local test environment did not have Open3D installed, so the unused import was stubbed for these no-point-cloud test paths; CI installs the project dependency set.

AI assistance

AI-assisted repository analysis and test planning were used. I reviewed the final diff and ran the validations above locally.