Remove usage of contextmanager in plugins for accessing connections in Airflow >= 3.1.2

[pankajkoti]

This PR adds explicit Airflow version gating and resilient connection handling for the Airflow 3 FastAPI plugin, along with tests and documentation updates. We now support the plugin only on Airflow >= 3.1 (enforced via a runtime check), move FastAPI app initialisation into the plugin’s constructor to avoid import-time failures, and conditionally use the connection_env context manager only on Airflow 3.1.0–3.1.1 (direct connection access on 3.1.2+). Tests cover the version gate, plugin initialisation, conditional connection handling, and functional routes (skipped on Airflow < 3.1). The documentation is updated to clearly state the Airflow >= 3.1 requirement and configuration keys for dbt_docs_projects.

closes: #2061

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.78%. Comparing base (67a53af) to head (0e5b407).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2073   +/-   ##
=======================================
  Coverage   97.77%   97.78%           
=======================================
  Files          91       91           
  Lines        5848     5862   +14     
=======================================
+ Hits         5718     5732   +14     
  Misses        130      130           

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[Copilot]

Pull Request Overview

This PR adds explicit Airflow version gating (>= 3.1) and resilient connection handling for the Cosmos Airflow 3 FastAPI plugin. The implementation includes runtime version checks, conditional use of the connection_env context manager (only for Airflow 3.1.0-3.1.1), and comprehensive test coverage.

Key changes:

• Enforces Airflow >= 3.1 requirement through runtime checks in plugin initialization and app creation
• Conditionally uses connection_env context manager only for Airflow 3.1.0-3.1.1 (direct connection access for 3.1.2+)
• Moves FastAPI app initialization from class-level to constructor to enable version validation before initialization

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
cosmos/plugin/airflow3.py	Adds version gating logic, conditional connection handling based on Airflow version, and moves FastAPI app initialization to constructor
tests/plugin/test_plugin_af3.py	Adds comprehensive tests for version gating, plugin initialization, conditional connection handling, and marks existing functional tests to skip on Airflow < 3.1
docs/configuration/hosting-docs.rst	Documents the Airflow >= 3.1 requirement for the plugin
docs/configuration/cosmos-conf.rst	Updates dbt_docs_projects description to clarify Airflow >= 3.1 requirement
docs/airflow3_compatibility/index.rst	Adds prominent note about Airflow >= 3.1 requirement for the FastAPI plugin

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[tatiana]

Looks excellent, @pankajkoti , thank you very much!
Minor comments inline.

[tatiana]

Looks great, @pankajkoti , thank you!