Add constrained extended nonlocal games support for SDP upper bounds and reproduce constrained BB84

[vprusso]

toqito.nonlocal_games.ExtendedNonlocalGame already handles standard extended nonlocal games, but it does not yet have a way to represent the constrained variant studied in Escolà-Farràs and Speelman, Lossy-and-Constrained Extended Non-Local Games with Applications to Quantum Cryptography (arXiv:2405.13717).

I do not want this issue to turn into "implement the whole paper." The target here is much narrower:

• add support for linear constraints on answer events in the SDP upper-bound routine for extended nonlocal games;
• use that support to reproduce the constrained BB84 monogamy-of-entanglement example from the paper.

Relevant code:

• toqito/nonlocal_games/extended_nonlocal_game.py
• the current SDP upper bound lives in ExtendedNonlocalGame.commuting_measurement_value_upper_bound
• existing tests live in toqito/nonlocal_games/tests/test_extended_nonlocal_game.py

What should be added:

• a way to specify linear constraints on answer-event probabilities for an ExtendedNonlocalGame;
• support for those constraints in the SDP upper-bound path;
• at least one helper or test fixture for the constrained BB84 game from the paper;
• regression tests covering the constrained BB84 case.

Suggested scope:

• it is fine to limit this to the upper-bound SDP path for now;
• it is fine if the initial interface is fairly simple, as long as it is documented and testable;
• do not try to solve the full lossy case in this issue.

Acceptance criteria:

• existing extended nonlocal game tests continue to pass;
• the new constrained BB84 tests pass;
• the implementation reproduces the expected unconstrained BB84 value as a sanity check;
• the PR explains clearly how the paper’s constraints were translated into the toqito SDP formulation.

Out of scope:

• full lossy-and-constrained game support;
• reproducing every application from the paper;
• refactoring unrelated nonlocal game code.

A good PR for this issue should include a short note pointing to the part of the paper being implemented and explaining the mapping from the paper’s notation to the variables used in toqito.