Copy target coordinates onto regridded result cube (#6844)

[gaoflow]

🚀 Pull Request

Description

Closes #6844.

After regridding, the horizontal (X/Y) coordinates on the result cube were the same objects as the coordinates on the target grid cube, so mutating one mutated the other:

result = src_cube.regrid(grid_cube, AreaWeighted())
result.coord("longitude") is grid_cube.coord("longitude")   # True!
result.coord("longitude").points += 100
grid_cube.coord("longitude").points                         # also changed!

Cause

iris.analysis._regrid._create_cube builds the result cube and attaches the target's coordinates. Its docstring already states:

Horizontal coordinates are copied from the target cube.

…and the source coordinates it carries across are indeed copied (coord.copy()). But the target coordinates were added by reference:

for tgt_coord, dim in zip(tgt_coords, (grid_dim_x, grid_dim_y)):
    ...
    result.add_dim_coord(tgt_coord, dim)   # same object as the target's

This affects all regridders that share _create_cube. (The curvilinear caller happened to pre-copy its coords, but AreaWeighted and the rectilinear RectilinearRegridder did not — and a regridder also reuses a single target snapshot across calls, so the aliasing could leak between successive results too.)

Fix

Copy each target coordinate inside _create_cube before adding it, matching both the docstring and the existing treatment of source coordinates. This centralises the behaviour for every regridder. The pre-existing .copy() in the curvilinear caller becomes harmlessly redundant.

Verification

• New test Test::test_result_coords_independent_of_target — asserts the result's lat/lon equal the target's but are distinct objects, and that mutating the result leaves the target unchanged. It fails on main and passes here.
• Manually confirmed for AreaWeighted, Linear and Nearest: result coordinates carry the correct points/bounds yet are independent of the target.
• tests/unit/analysis/regrid/ and tests/unit/analysis/area_weighted/ suites pass (81 tests; the 4 Test__derived_coord errors are pre-existing and unrelated — they require optional regridding dependencies not installed locally).
• ruff check / ruff format clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.15%. Comparing base (746f0a6) to head (fdd27dd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7124   +/-   ##
=======================================
  Coverage   90.15%   90.15%           
=======================================
  Files          91       91           
  Lines       24985    24986    +1     
  Branches     4685     4685           
=======================================
+ Hits        22526    22527    +1     
  Misses       1682     1682           
  Partials      777      777           

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[ukmo-ccbunney]

Thanks for the fix @gaoflow - code changes look good. 👍🏼

As mentioned in some of your other PRs:
There are some updates required regarding the What's New, but I see that you have already been applying those to your other PRs - top points for that! 💯

Are you happy to make the same updates here?
If not, please do say and I'll raise a PR against your branch.

[gaoflow]

Updated this PR for the new towncrier changelog layout and rebased onto current main.

Changes in fdd27dde:

• removed the hand-written docs/src/whatsnew/latest.rst entry from this PR
• added changelog/7124.bugfix.rst
• kept the regrid code/test change otherwise unchanged

Verification after the rebase:

ruff check lib/iris/analysis/_regrid.py lib/iris/tests/unit/analysis/area_weighted/test_AreaWeightedRegridder.py
ruff format --check lib/iris/analysis/_regrid.py lib/iris/tests/unit/analysis/area_weighted/test_AreaWeightedRegridder.py
uv run --with pytest --with pytest-mock --with requests --with filelock --with-editable . python -m pytest lib/iris/tests/unit/analysis/area_weighted/test_AreaWeightedRegridder.py -q
uv run --with towncrier towncrier build --draft --version 0.0
git diff --check origin/main...HEAD

The targeted test file passed with 9 passed, 2 skipped; the skips are the existing external-data tests.

[ukmo-ccbunney]

Thank you @gaoflow for taking the time to rebase you branch and add the news fragment. 🏅

Changes look good - LGTM! 🚀