CI: fail fast on unresolved merge markers in uv.lock

[Copilot]

What problem this PR addreseses

The ubuntu-latest / python 3.12 Actions job fails at dependency install because uv sync --locked cannot parse uv.lock when conflict markers are present. This PR adds an explicit preflight check so the workflow fails immediately with a clear diagnostic.

• Change
  • Lockfile guardrail: added a Validate uv.lock step in .github/workflows/uv-pytests.yml before install.
  • Failure mode clarity: workflow now reports unresolved merge markers directly instead of surfacing a downstream TOML parser failure.

- name: Validate uv.lock
  run: |
    if grep -nE '^(<<<<<<<|=======|>>>>>>>)' uv.lock; then
      echo "Unresolved merge markers found in uv.lock"
      exit 1
    fi

What should the reviewer(s) do

Review the new pre-install validation step in the pytest workflow and confirm the fail-fast behavior/error message is appropriate for lockfile integrity checks.

• This PR introduces a public-facing change (e.g., figures, CLI input/output, API).
  • For functional and algorithmic changes, tests are written or updated.
  • Documentation (e.g., tutorials, examples, README) has been updated.
  • A tracking issue or plan to update documentation exists.
• This PR affects internal functionality only (no user-facing change).

[bobleesj]

pretty cool that copilot can use PR template

[cophus]

@copilot who made this PR?

[henrygbell]

@cophus I think I accidentally prompted this. I clicked "ask copilot to fix this" for an error. I am not sure we want this change.

[gvarnavi]

What was the error? We definitely don't want a situation where uv.lock has merge conflicts, but presumably the weekly cron job updating uv.lock should prevent devs from having to think much about the lock file.

[henrygbell]

The error I get is in this PR: #204 in the python 3.12 project installation:

Run uv sync --locked --group test
Using CPython 3.12.3 interpreter at: /usr/bin/python3.12
Creating virtual environment at: .venv
error: Failed to parse `uv.lock`
  Caused by: TOML parse error at line 20, column 9
   |
20 | <<<<<<< HEAD
   |         ^
key with no value, expected `=`
Error: Process completed with exit code 2.

More details here:
https://github.com/electronmicroscopy/quantem/actions/runs/26850270732/job/79180346028?pr=204

[henrygbell]

PR closed, I did not mean to create it. Problem with uv.lock has been solved