Validate provided scheduler before qompile

[masa10-f]

Before submitting, please check the following:

• Make sure you have tests for the new code and that test passes (run pytest)
• If applicable, add a line to the [unreleased] part of CHANGELOG.md, following keep-a-changelog.
• Format added code by ruff
• Type checking by mypy and pyright
• Make sure the checks (github actions) pass.
• Check that the docs compile without errors (run make html in ./docs/ - you may need to install dependency for sphinx docs, see docs/requirements.txt.)

Then, please fill in below:

Context (if applicable):

qompile() accepted user-provided Scheduler instances without validating their schedules first, so invalid manual schedules could reach pattern generation and fail later or produce incorrect command timing.

Description of the change:

• Validate a provided scheduler in _qompile() before reading its timeline.
• Keep the existing internal scheduler path unchanged when no scheduler is provided.
• Add a regression test covering DAG-violating scheduler input to qompile().
• Record the fix in the Unreleased changelog.

Validation performed locally:

• python -m ruff check ./graphqomb ./tests ./examples
• python -m ruff format --check --diff
• python -m pytest -m "not pyzx"
• python -m pytest -m pyzx
• python -m pytest --cov=graphqomb --cov-report=term-missing --cov-report=xml --cov-branch
• python -m mypy
• pyright
• env PRE_COMMIT_HOME=/tmp/pre-commit pre-commit run --all-files
• sphinx-build -W docs/source docs/build

Related issue:

Closes #154

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.13%. Comparing base (54d917d) to head (4f1270c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #199   +/-   ##
=======================================
  Coverage   85.13%   85.13%           
=======================================
  Files          24       24           
  Lines        3242     3243    +1     
  Branches      563      563           
=======================================
+ Hits         2760     2761    +1     
  Misses        352      352           
  Partials      130      130           

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[masa10-f]

@codex review

[chatgpt-codex-connector]

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