[hotfix] Move chat-model os.environ mutations out of module scope in e2e tests

[weiqingy]

Purpose of change

os.environ[X] = Y mutations at module scope in e2e test files leak into the rest of the pytest process at collection time. The mechanism:

• tools/ut.sh runs pytest flink_agents -k "not e2e_tests".
• -k only filters which tests execute, not which files are collected. Pytest imports every matching test file during the collection phase.
• Importing an e2e module that does os.environ[X] = Y at module scope executes the assignment as a side effect, polluting X for every later-collected test (e.g. the integrations/ tests).

PR #686 fixed this for OLLAMA_EMBEDDING_MODEL. This PR covers the same pattern for the chat-model-side env vars.

Files fixed

File	Env var(s) moved out of module scope
e2e_tests_resource_cross_language/chat_model_cross_language_test.py:44	OLLAMA_CHAT_MODEL
e2e_tests_resource_cross_language/yaml_cross_language_test.py:72	OLLAMA_CHAT_MODEL
e2e_tests_integration/chat_model_integration_test.py:32,34,36,38,42	TONGYI_CHAT_MODEL, OLLAMA_CHAT_MODEL, OPENAI_CHAT_MODEL, AZURE_OPENAI_CHAT_MODEL, AZURE_OPENAI_API_VERSION
e2e_tests_integration/react_agent_test.py:54	OLLAMA_CHAT_MODEL

Approach

Used monkeypatch.setenv(...) inside the test function bodies. This:

• Sets the env var only when the test actually runs (no collection-time pollution)
• Auto-restores the original value after the test (no inter-test leakage either)
• Preserves runtime propagation to the Flink subprocess

In chat_model_integration_test.py also converted the already-inside-function os.environ["MODEL_PROVIDER"] = model_provider to monkeypatch.setenv for consistency — otherwise the parametrize iterations (Ollama / Tongyi / OpenAI / AzureOpenAI) would leak MODEL_PROVIDER between runs.

Tests

• ./tools/lint.sh -c clean
• AST parse of all 4 files clean
• The actual proof comes when CI runs and the chat-model tests no longer pollute the integrations test process. (No new test added because the underlying behavior — running an e2e test — is unchanged; the fix is purely about test-process hygiene.)

API

No API changes — test files only.

Documentation

• doc-needed
• doc-not-needed
• doc-included

[wenjin272]

LGTM