CI hardening: Python 3.13/3.14, format checks, lint tests/, mypy on all versions, coverage floor

[adaumsilva]

Description

.github/workflows/ci.yml currently:

• tests only Python 3.10–3.12 (3.13 has been stable for a year; contributors are already on 3.14)
• runs ruff check ragframework/ only — tests/ and examples/ are never linted
• never runs black --check or isort --check-only although both are in [dev] and documented in CONTRIBUTING.md
• runs mypy only on 3.10
• uploads coverage but enforces no minimum (fail_ci_if_error: false, no --cov-fail-under)
• does not install the [pdf] extra, so PDFLoader is only exercised through a fake pypdf

Acceptance criteria

• Matrix extended to ["3.10", "3.11", "3.12", "3.13", "3.14"]; classifiers in pyproject.toml updated to match; black/ruff target-version lists updated
• Separate fast lint job: ruff check . plus either ruff format --check . or black --check . && isort --check-only . (pick one formatter; if ruff-format, drop black/isort from [dev] and update CONTRIBUTING.md)
• mypy ragframework/ runs on the oldest and newest Python in the matrix
• pytest --cov-fail-under=85 (measure the current number first and set the floor just below it)
• [pdf] added to the CI install so at least one real-pypdf test can run (a tiny generated PDF fixture via pypdf.PdfWriter)
• concurrency: block added so superseded PR runs are cancelled
• Any failures the wider lint surfaces are fixed in the same PR

Files to touch

• .github/workflows/ci.yml
• pyproject.toml
• CONTRIBUTING.md
• possibly tests/ and examples/ (lint fixes)

Resources

• actions/setup-python
• pytest-cov --cov-fail-under

Estimated effort: Medium (half day)

[sanjana658]

Hi @adaumsilva, I'd like to work on this issue. I have experience with Python, testing, linting, and CI workflows, and I'd be happy to implement the requested CI improvements. Could you please assign this issue to me?

[adaumsilva]

Assigned to you @sanjana658