Add tests for MixedBehaviorProfile regrets (Resolves #707)

[satyam102006]

Description

This PR resolves #707 by adding the missing test coverage for MixedBehaviorProfile regret calculations. This aligns the behavior profile tests with the recent strategy profile updates introduced in #642 and #617.

Changes Made

• Added test_mixed_behavior_profile_max_regret to tests/test_behav.py.
• Added test_mixed_behavior_profile_agent_max_regret to tests/test_behav.py.
• Parameterized the tests to validate both standard floating-point (rational=False) and exact rational (rational=True) execution using the existing games fixtures.
• Asserted that both methods return the correct data types (float vs gbt.Rational) and valid non-negative values.

Testing

• Ran the full test suite locally via pytest. All 564 tests passed successfully.

Closes #707

[satyam102006]

Co-authored-by: @SammY-000

[tturocy]

We are not able to review unsolicited pull requests that have not been previously discussed with a developer.

If you are interested in participating in GSoC 2026, please read https://www.gambit-project.org/gsoc_2026/ and specifically "Contributor Guidance". With that information we can assess whether you are likely a good candidate to work on Gambit and provide with suggestions and guidance on initial issues to try out.

[satyam102006]

Hi @tturocy , I completely understand why my PR was closed I apologize for jumping the gun before discussing it! I am used to a very fast-paced workflow in other open-source projects and was just eager to dive into the Gambit codebase.

I have formally sent my CV and Cover Letter to your email as per the GSoC Contributor Guidance. Once you have had a chance to review my profile, I would love to discuss the proper approach to solving this issue (or any other issue you recommend) to officially evaluate my technical fit. Looking forward to hearing from you