[FSTORE-1962] Automatically add feature group topics with "_onlinefs" sufix if users provide a topic that does not end with "onlinefs"

[manu-sj]

This PR automatically adds the '_onlinefs' prefix to topic provided to a feature group so that online fs can process it.

JIRA Issue: https://hopsworks.atlassian.net/browse/FSTORE-1962

Priority for Review: -

Related PRs: -

How Has This Been Tested?

• Unit Tests
• Integration Tests
• Manual Tests on VM

Checklist For The Assigned Reviewer:

- [ ] Checked if merge conflicts with master exist
- [ ] Checked if stylechecks for Java and Python pass
- [ ] Checked if all docstrings were added and/or updated appropriately
- [ ] Ran spellcheck on docstring
- [ ] Checked if guides & concepts need to be updated
- [ ] Checked if naming conventions for parameters and variables were followed
- [ ] Checked if private methods are properly declared and used
- [ ] Checked if hard-to-understand areas of code are commented
- [ ] Checked if tests are effective
- [ ] Built and deployed changes on dev VM and tested manually
- [x] (Checked if all type annotations were added and/or updated appropriately)

[Copilot]

Pull request overview

Adds automatic enforcement of the _onlinefs Kafka topic suffix for online-enabled feature groups, appending it when missing and warning the user.

Changes:

• Introduces topic name validation during feature group metadata save to append _onlinefs when required.
• Adds unit tests covering suffix auto-append, no-op when present, and None handling.
• Updates Python docstrings to document the _onlinefs suffix requirement and auto-append behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
python/hsfs/core/feature_group_engine.py	Adds _onlinefs suffix constant, validation/warning logic, and hooks validation into save path.
python/tests/core/test_feature_group_engine.py	Adds tests to ensure validation appends suffix + emits warning, and no warning for valid topics/None.
python/hsfs/feature_store.py	Updates feature group creation docstrings to document required suffix and auto-append behavior.
python/hsfs/feature_group.py	Updates constructor/property docstrings to document _onlinefs suffix requirement and auto-append behavior.

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[bubriks]

Not too related to this pr, but just FYI some users are asking for possibility to change fg topic after fg is created (right now only possibly by running sql commands, this would be new jira so not an issue).

[bubriks]

by default onlinefs service consumes based on matching suffix, but it can be also made to consume from specific topics (without using _onlinefs suffix)

Not sure if used currently, but depending on the user it might be important

[bubriks]

right now user topics are generated like this: https://github.com/logicalclocks/hopsworks-ee/blob/ffb374fd90b5c98d65c2e12191f29351ad4be5c1/hopsworks-common/src/main/java/io/hops/hopsworks/common/hdfs/Utils.java#L191

basicaly backend decides what should be returned to the user, maybe same should be done here. (aka. do we want user provided topic to be explicit or implicit)

[bubriks]

if we correct maybe we should allow user to disable corrections? in case they want to use that exact name without suffix.