[ENH]: Make PinballLoss missing alpha error message more informative

[MOHITKOURAV01]

Reference Issues/PRs

Fixes #977

What does this implement/fix? Explain your changes.

This PR enhances the ValueError message raised by the PinballLoss metric when one or more requested alpha quantiles are missing from the given predictions (y_pred).

Previously, the error message was generic and did not specify which alphas were missing, requiring manual debugging. This resolves an active TODO in skpro/metrics/_classes.py to make the error message more informative.

Changes:

• Modified PinballLoss._evaluate_by_index to compute the set difference between requested alpha and available y_pred_alphas.
• Updated the error string to explicitly list the missing values.

• Old Error Message: ValueError: "not all quantile values in alpha are available in y_pred"
• New Error Message: ValueError: "not all quantile values in alpha are available in y_pred. Missing alphas: [0.1, 0.9]"

Does your contribution introduce a new dependency? If yes, which one?

No.

What should a reviewer concentrate their feedback on?

• The string formatting of the new ValueError message in PinballLoss.
• The logic used to identify missing alphas (set difference).

Did you add any tests for the change?

Yes, I have updated test_evaluate_alpha_negative in test_probabilistic_metrics.py to use pytest.raises(ValueError, match=".*Missing alphas:.*"). This ensures the new informative message is correctly triggered and verified.

Any other comments?

All probabilistic metrics tests passed successfully on my local machine using pytest.

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