Support cancelling a submission

[jerbaroo]

This PR adds a new method cancel_submission to ProducerClient. This method takes a SubmissionId and returns None if successful. If the submission could not be found a SubmissionNotFoundError is raised. If the submission was already completed, failed or cancelled then a SubmissionNotCancellableError is raised which contains the specific reason.

[Copilot]

Pull request overview

This PR adds end-to-end support for cancelling an in-progress submission: it introduces a cancelled submission state persisted in a new submissions_cancelled table, exposes a new producer HTTP endpoint + Rust client method, and wires the feature through the Python bindings and tests.

Changes:

• Add SubmissionStatus::Cancelled / SubmissionCancelled and a DB cancel path that moves submissions into submissions_cancelled.
• Expose POST /producer/submissions/cancel/{submission_id} and a matching Client::cancel_submission method.
• Add Python API + exceptions for cancellation and add roundtrip tests for cancel/not-found/not-cancellable cases.

Reviewed changes

Copilot reviewed 14 out of 16 changed files in this pull request and generated 12 comments.

Show a summary per file

File	Description
opsqueue/src/prometheus.rs	Adds metric names for cancelled submissions.
opsqueue/src/producer/server.rs	Adds new HTTP route/handler for cancelling submissions.
opsqueue/src/producer/client.rs	Adds client method to call the cancel endpoint and decode errors.
opsqueue/src/common/submission.rs	Adds cancelled submission status, DB queries, cancel mutation, and cleanup logic.
opsqueue/src/common/errors.rs	Adds serializable cancellation-related error types.
opsqueue/opsqueue_example_database_schema.db	Adds an example SQLite schema DB file reflecting new tables.
opsqueue/migrations/20260225164600_submissions_cancelled.sql	Adds migration creating submissions_cancelled.
libs/opsqueue_python/tests/test_roundtrip.py	Adds Python integration tests for cancel behavior and exceptions.
libs/opsqueue_python/src/producer.rs	Exposes cancel_submission from Rust -> Python client.
libs/opsqueue_python/src/lib.rs	Registers new Python-exposed types (but see review comments).
libs/opsqueue_python/src/errors.rs	Maps new cancellation errors into Python exceptions.
libs/opsqueue_python/src/common.rs	Adds Python types for cancelled status and not-cancellable details.
libs/opsqueue_python/python/opsqueue/producer.py	Adds Python ProducerClient.cancel_submission wrapper + docs.
libs/opsqueue_python/python/opsqueue/exceptions.py	Adds Python SubmissionNotCancellableError and enriches SubmissionNotFoundError.
Cargo.toml	Bumps workspace version to 0.35.0.
Cargo.lock	Updates locked versions for opsqueue and opsqueue_python.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Pull request overview

Copilot reviewed 14 out of 16 changed files in this pull request and generated 6 comments.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

You can also share your feedback on Copilot code review. Take the survey.

[jerbaroo]

If the client is requesting to cancel a submission, they will know the ID of the submission they are trying to cancel. So I suggest this change is reverted.