[tmva][sofie] Guard old Keras parser against Keras-3

[Shlok-Saxena]

This Pull request

Adds a defensive runtime guard to the TMVA SOFIE Keras parser to prevent segmentation faults when used with Keras 3 (enabled by default in TensorFlow ≥ 2.16).

The current parser is implemented against the Keras 2 API. When Keras 3 objects are encountered, undefined behavior may occur, leading to runtime crashes. This PR does not add Keras 3 support; it only detects the unsupported configuration early and fails cleanly with a clear error message.

This provides a temporary safety measure while proper Keras 3 support is being developed.

Changes or fixes:

• Add an explicit runtime check for the Keras major version
• Disable the old SOFIE Keras parser when Keras ≥ 3 is detected
• Replace segmentation faults with a clear, user-facing error message
• Do not change behavior for Keras 2 users

Checklist:

• tested changes locally

This PR fixes #20591

[guitargeek]

Thank you very much for the PR! This is a good temporary solution. Just a few comments:

• please remove the commit with the code formatting. It's not worth to format code that we plan to delete in #19692 anyway, so that's just bloating the history. You can ignore the clang-format CI errors this time.
• I think you don't need to explicitly import keras again
• Please drop the first commit, which comes from an unrelated development for which you have already opened a separate PR.

[github-actions]

Test Results

    18 files      18 suites   3d 3h 41m 48s ⏱️
 3 792 tests  3 788 ✅ 0 💤 4 ❌
65 369 runs  65 365 ✅ 0 💤 4 ❌

For more details on these failures, see this check.

Results for commit 60975ed.

♻️ This comment has been updated with latest results.

[guitargeek]

Thank you very much!