Fix bilinear ABL grid_start probe/nozzle coordinate mixup

[PenguinAkiko]

Description

This PR fixes a critical coordinate-space mixup for the bedlevel.grid_start position when using Bilinear Auto Bed Leveling (AUTO_BED_LEVELING_BILINEAR).

The Problem:

The bilinear ABL probing process builds its mesh in probe-space, where abl.probe_position_lf represents the coordinate directly under the probe. However, the resulting bedlevel.grid_start value is later consumed by other systems (like bedlevel.get_z_correction()) as a motion/nozzle-space origin.

On machines with a non-zero NOZZLE_TO_PROBE_OFFSET in the XY plane, this discrepancy caused the entire leveling mesh to be incorrectly shifted by the probe's XY offset, leading to inaccurate bed compensation.

The Solution:

To resolve this, this PR introduces a centralized utility function, Probe::convert_to_nozzle_xy(), to handle the conversion from probe-space to nozzle-space. This function is now used in two key locations:

1. In G29: The bilinear grid origin is now converted to nozzle-space before it is compared with a stored mesh or saved with bedlevel.set_grid().
2. In M420 S2: The test pattern generator is also updated to use the same conversion, ensuring that all methods of generating a bilinear mesh use consistent, nozzle-space coordinates for the grid origin.

This change has no functional effect on setups where the XY probe offset is zero or when using NOZZLE_AS_PROBE.

Requirements

Any 3D printer configured with AUTO_BED_LEVELING_BILINEAR.

Benefits

Fixes a bug that incorrectly shifted the entire Bilinear ABL mesh by the probe's XY offset.

Configurations

No configuration files are attached as the change is hardware-independent.

To test this PR, any configuration with the following enabled will reproduce the issue and verify the fix:

// in Configuration.h
#define AUTO_BED_LEVELING_BILINEAR

// A probe with an XY offset from the nozzle
#define NOZZLE_TO_PROBE_OFFSET { 10, -5, 0 }

Related Issues

Fixes a bug that does not appear to have a corresponding open issue.