ci(devcontainer): Debian 13 Trixie + Python 3.14, Ruff settings, drop .vscode (keeper-safe .json-only)

[priya-sundaram-dev]

Describe your change:

Upgrades .devcontainer to Debian 13 "Trixie" + Python 3.14 and modernizes the VS Code settings. Replaces #15160 / #15161.

algorithms-keeper auto-closed #15160 because it edited the extension-less .devcontainer/Dockerfile (the keeper's ACCEPTED_EXTENSIONS allow-list has no entry for Dockerfile, so reopening re-triggers the close). This version touches .json files only, and this time includes the filled checklist (that's why #15161 was auto-closed).

Same outcome, keeper-safe:

• Debian 13 "Trixie" + Python 3.14: bump build.args.VARIANT 3.13-bookworm -> 3.14-trixie. The base image is selected entirely from devcontainer.json — the Dockerfile's ARG VARIANT reads it — so the Dockerfile is left untouched and keeps installing requirements.txt + pipx pre-commit/ruff.
• VS Code settings modernized: the old python.linting.* / python.formatting.blackPath keys were removed by the Python extension long ago. Replaced with Ruff formatOnSave + source.fixAll/source.organizeImports, matching pre-commit and CI. Added the charliermarsh.ruff extension.
• .vscode/ deleted (per your preference on priya-sundaram-dev as a maintainer? #15081): its sole file only muted a PR-branch notification; pre-commit/Ruff cover the rest.

Open questions from #15081:

• Latest-and-greatest CPython: there is no floating 3-trixie tag on the mcr.microsoft.com/vscode/devcontainers/python path, so VARIANT must name the minor version; bump it when a new stable ships.
• Free-threading (3.14t): these images don't publish t variants and build args can't read .python-version, so 3.14t can't be selected via the tag alone — it would need a Dockerfile/uv change (which the keeper blocks from me). Happy to do that as a maintainer-merged follow-up.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[priya-sundaram-dev]

Heads-up: the `Test DevContainer Build` check was red, and it turned out to be a pre-existing breakage on main, not something this PR introduced.

Root cause: `.devcontainer/Dockerfile` does `COPY requirements.txt /tmp/pip-tmp/`, but `requirements.txt` was removed when the repo moved its dependencies to `pyproject.toml`. The image build has been broken since that migration — it just never surfaced because CI only runs on `.devcontainer/**` changes and nobody had touched that directory.

Fix (pushed just now, still `.json`-only so it stays keeper-safe): drop the local Dockerfile build and pull the upstream mcr.microsoft.com/devcontainers/python:3.14-trixie image instead, installing pre-commit + ruff + uv in postCreateCommand (which then runs the existing post_install). That removes the requirements.txt dependency entirely and is less to maintain going forward.

The .devcontainer/Dockerfile is now unused. I left it in place because it is an extension-less filename the keeper still auto-closes; it can be deleted in a quick follow-up once #228 (which whitelists the Dockerfile filename) lands. Watching the new CI run.

[cclauss]

  [2026-09-02T19:48:03.349Z]   installed package pre-commit 4.6.2, installed using Python 3.14.7
    These apps are now available
      - pre-commit
    installed package ruff 0.16.5, installed using Python 3.14.7
    These apps are now available
      - ruff
    installed package uv 0.12.9, installed using Python 3.14.7
    These apps are now available
      - uv
      - uvx