[BUG] Fix wrong chi-squared identity in ChiSquared._energy_x

[ANANYA542]

###Description
The closed-form _energy_x formula used chi2.cdf(xi, k + 1) for the partial expectation integral, but the correct identity is chi2.cdf(xi, k + 2).
This PR fixes the identity and updates the associated docstrings.

This caused silently wrong CRPS/energy scores with errors up to 48%. The fix changes k+1 to k+2 on line 186 and updates both docstrings.

Reference Issues/PRs

fixes #964

What does this implement/fix? Explain your changes.

The chi-squared probability density function satisfies the identity:
t · f_χ²(t; k) = k · f_χ²(t; k + 2)

This is easily proved using the relation Γ(k/2 + 1) = (k/2) · Γ(k/2). Integrating both sides yields:
∫₀ˣ t · f_χ²(t; k) dt = k · F_χ²(x; k+2)

The skpro implementation on line 186 mistakenly used k + 1 for this integral: cdf_k1 = chi2.cdf(xi, k + 1).
This mathematical error produced silently incorrect CRPS/energy scores with errors ranging from 25% to nearly 50%.
Changes in this PR:

1. Changed cdf_k1 = chi2.cdf(xi, k + 1) to cdf_k2 = chi2.cdf(xi, k + 2).
2. Updated the class-level docstring to reflect the correct k+2 formula.
3. Updated the _energy_x method docstring.
   Verification:
   I created a verification script that compares the fixed _energy_x against direct numerical integration for various values of k and x. All errors are now at machine precision (< 1e-11).
   The screenshot depicting the entire verification has been attatched below:

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

No.

What should a reviewer concentrate their feedback on?

• Verifying the mathematical identity (standard chi-squared partial expectation formula).
• Ensuring the docstring math formatting renders correctly.

Did you add any tests for the change?

No explicit new tests added, but the mathematical fix makes the closed-form solution match the numerical ground truth.

Any other comments?

PR checklist

For all contributions

• I've added myself to the list of contributors with any new badges I've earned :-)
  How to: add yourself to the all-contributors file in the skpro root directory (not the CONTRIBUTORS.md). Common badges: code - fixing a bug, or adding code logic. doc - writing or improving documentation or docstrings. bug - reporting or diagnosing a bug (get this plus code if you also fixed the bug in the PR).maintenance - CI, test framework, release.
  See here for full badge reference
• The PR title starts with either [ENH], [MNT], [DOC], or [BUG]. [BUG] - bugfix, [MNT] - CI, test framework, [ENH] - adding or improving code, [DOC] - writing or improving documentation or docstrings.

For new estimators

• I've added the estimator to the API reference - in docs/source/api_reference/taskname.rst, follow the pattern.
• I've added one or more illustrative usage examples to the docstring, in a pydocstyle compliant Examples section.
• If the estimator relies on a soft dependency, I've set the python_dependencies tag and ensured
  dependency isolation, see the estimator dependencies guide.