test: standardize all test filenames to _test.py postfix (#760)

[Raja-89]

Standardize all test filenames across tests/unit/ and tests/integration/ to consistently use the _test.py postfix.

This improves test discovery, simplifies CI enforcement, and aligns with the new naming workflow requirements.

Changes included:

• Rename all unit test files from test_.py → _test.py
• Rename integration test files that did not already follow _test.py
• Preserve history using git mv

Ensure no changes are made to helper/config files:

__init__.py

conftest.py

mock_server.py

utils_for_test.py

Updated imports where necessary

Added entry to UNRELEASED changelog

Related issue(s)

Fixes #760

Notes for reviewer

• All renamed files were moved using git mv to preserve file history.
• Entire test suite was executed locally — all tests pass.
• Naming workflow now correctly validates all files.
• No functional code changes were made; this PR is naming-only.

Checklist

~Documented (changelog entry added)
~Tested (full test suite run locally; naming CI check passes)

[exploreriii]

Hi @Raja-89 please resolve conflicts 👍
You'll need to rebase
read rebasing.md and merge_conflicts.md please
it looks like your origin may not in line with upstream

[exploreriii]

Hi @Raja-89 please get in touch if you need help, see MAINTAINERS.md

[exploreriii]

Merge conflicts require fixing - please get in touch if you need help