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

[priya-sundaram-dev]

Replaces #15160, which algorithms-keeper kept auto-closing 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 — that's why the reopen at 10:50 didn't stick).

This version touches .json files only, so the keeper accepts it, while delivering the same outcome:

• 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): the sole file only muted a PR-branch notification; pre-commit/Ruff cover the rest.

Notes for the open questions on #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. (I added a comment saying so.)
• Free-threading (3.14t): these images don't publish t variants, and devcontainer 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.

Supersedes #15160.

[algorithms-keeper]

Closing this pull request as invalid

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