Add fidelity and isclose methods to Statevec

[CodeMaverick2]

Before submitting, please check the following:

• Make sure you have tests for the new code and that test passes (run nox)
• If applicable, add a line to the [unreleased] part of CHANGELOG.md, following keep-a-changelog.
• Format added code by ruff
  • See CONTRIBUTING.md for more details
• Make sure the checks (github actions) pass.

Then, please fill in below:

Context (if applicable):

• Tests across the codebase manually compute pure-state fidelity with np.abs(np.dot(state.flatten().conjugate(), state2.flatten())) == pytest.approx(1) in ~68 places.

Description of the change:

• Add Statevec.fidelity(other) computing |⟨ψ₁|ψ₂⟩|² and Statevec.isclose(other) checking equality up to global phase
• Replace ~68 manual inner-product assertions across 11 test files with isclose
• Add 9 unit tests for the new methods

Related issue:
Closes #386

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.74%. Comparing base (c1f689d) to head (4cb7535).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #433   +/-   ##
=======================================
  Coverage   88.73%   88.74%           
=======================================
  Files          44       44           
  Lines        6303     6308    +5     
=======================================
+ Hits         5593     5598    +5     
  Misses        710      710           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[emlynsg]

Great work, thank you very much.

We at TeamGraphix have a 2 approval policy, so when a second maintainer approves this PR we are happy to merge it and close the issue with you allocated.

[CodeMaverick2]

Great work, thank you very much.

We at TeamGraphix have a 2 approval policy, so when a second maintainer approves this PR we are happy to merge it and close the issue with you allocated.

@emlynsg
Thanks for the approval!
Since TeamGraphix follows a 2-approval policy, could you please tag another maintainer to take a look at the PR so we can move it forward?

[matulni]

Hi @CodeMaverick2, thanks for your contribution and welcome to the project!

PR looks good, I made a couple of small comments and I'll be happy to approve (and assign you the issue) once they've been addressed. Please, don't forget to update the CHANGELOG and resolve any conflicts arising from recent merges.

Thanks!

[matulni]

Thanks, LGTM!