docs(contributing): align test and development instructions

[yousseffaouzi]

What was broken

The contributor guide and PR templates used test paths that do not exist in the current repository:

• uv run pytest tests/unit
• uv run pytest tests/integration
• ./scripts/run_tests.sh

The tests actually live under src/, and INSTRUCTIONS.md already documents the supported test selection. CONTRIBUTING.md also assumed Ruff was available through the development dependencies even though it is not declared in the current pyproject.toml.

Changes

• Use uv run pytest src/ -k "not integration" for the service-free test selection.
• Use uv run pytest src/ for the full suite, subject to documented integration prerequisites.
• Remove documentation for the nonexistent scripts/run_tests.sh script.
• Document uvx ruff and note that whole-repository checks report existing unrelated diagnostics.
• Correct the feature and bugfix PR templates.

Scope: contributor documentation only. There are no benchmark behavior, scoring, baseline, or dependency changes. Existing PR #331 separately handles adding Ruff to development dependencies.

Verification

• uv sync --frozen passed.
• uv run --frozen pytest src/llm/tests src/servers/utilities/tests -q 33 passed, including re-running against the published branch.
• git diff --check upstream/main...docs/fix-contributor-test-instructions passed.
• uv run --frozen pytest src/ -k "not integration" collected 720 tests, with 13 deselected and 707 selected; terminated with exit code 137 in the available environment before completion. A full-suite pass is not claimed.
• uvx ruff check . 1,210 existing repository-wide findings.
• uvx ruff format --check . 84 existing files reported as needing reformatting. No Python code is changed in this PR.

The commit includes a DCO sign-off matching its GitHub author address.