PYTHON-6112 Use a stable set of tasks and and coverage in Evergreen

[blink1073]

PYTHON-6112

Changes in this PR

• Pins Evergreen PR tasks to fixed Python boundaries (min Python with min-deps, max Python and one free-threaded Python) instead of rotating them.
• Runs every PR task with COVERAGE=1 on the newest stable version of the server, and leaves mainline batch coverage on the full rotation (every CPython across all server versions, including the latest nightly).
• Unit tests assert the rules; the generate-config pre-commit hook regenerates the config.
• Also fixes the AWS ECS variant's leaked pr tag.

Test Plan

• New unit tests: pytest .evergreen/scripts/test_generate_config.py -m "".
• Evergreen PR tests.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)? Not needed; the change affects Evergreen configuration, docs, and its test script, not the driver.
• Is there test coverage?
• Is any followup work tracked in a JIRA ticket? If so, add link(s). No followup work.

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[Copilot]

🟡 Changes recommended

The generated tasks still lose parts of the batch rotation and nightly coverage, and some PR-tagged tasks do not meet the stated pinning or coverage rules.

8 open findings

• PR-tagged server tasks do not pin MAX_VERSION or preserve rotation · New
• Non-coverage companion incorrectly retains the pr tag · New
• Latest non-standard nightly coverage and companion are lost · New
• Pinned PR tasks replace rotated tasks and lose nightly coverage · New
• AWS ECS variants incorrectly inherit the macOS pr tag · New
• AWS coverage excludes rotated max-Python tasks · New
• ECDSA latest nightly task is replaced by a stable PR task · New
• Coverage test lacks assertions for PR and nightly task coverage · New

What changed in this PR

This PR aims to make Evergreen PR testing predictable by pinning Python and server boundaries while retaining mainline test rotation.

Changes:

• Pins selected PR tasks to stable Python and server versions and enables coverage for them.
• Adds generator tests and runs them before config generation.
• Updates contributor guidance and regenerates the task configuration.

File	Description
CONTRIBUTING.md	Documents the intended PR task rules.
.pre-commit-config.yaml	Adds test dependencies to the generation hook.
.evergreen/​scripts/​test_generate_config.py	Adds checks for generated PR tasks.
.evergreen/​scripts/​generate-config.sh	Runs those checks before generation.
.evergreen/​scripts/​generate_config.py	Changes task selection, tags, and coverage.
.evergreen/​scripts/​generate_config_utils.py	Defines Python and server boundaries.
.evergreen/​generated_configs/​tasks.yml	Records the generated task changes.

🧠 Review effort: Lite

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[Copilot]

🟡 Changes recommended

Server-version PR tasks still inherit each variant’s server version, and the new assertion does not detect that gap.

2 open findings

• PR-tagged server tasks do not pin MAX_VERSION or preserve rotation
• Test PR tasks inherit only MAX_VERSION · New

7 resolved since last review

• Coverage test lacks assertions for PR and nightly task coverage
• ECDSA latest nightly task is replaced by a stable PR task
• AWS coverage excludes rotated max-Python tasks
• AWS ECS variants incorrectly inherit the macOS pr tag
• Pinned PR tasks replace rotated tasks and lose nightly coverage
• Latest non-standard nightly coverage and companion are lost
• Non-coverage companion incorrectly retains the pr tag

🧠 Review effort: Lite

[Copilot]

🔵 Needs a closer look

Three unresolved moderate findings affect task selection and duplicate execution, plus one dependency-documentation nit.

0 open findings

2 resolved since last review

• PR-tagged server tasks do not pin MAX_VERSION or preserve rotation
• Test PR tasks inherit only MAX_VERSION

Previously missed (1)

In code that hasn't changed since last review

Document pytest and pytest-asyncio test dependencies

.evergreen/​scripts/​test_generate_config.py:5

The test now requires pytest (and the hook installs pytest and pytest-asyncio), but this docstring still lists only shrub.py and pyyaml. Update the dependency list so developers following the documented standalone command know what must be installed.

🧠 Review effort: Lite

[Copilot]

🔵 Needs a closer look

Three moderate findings and one nit remain unresolved.

0 open findings

🧠 Review effort: Lite