Fix qnode_spectrum with interface="auto"

[dwierichs]

Context:
An example from the docstring of qnode_spectrum is failing because the detection of trainable arguments fails when using Autograd via vanilla numpy inputs and interface="auto" (default).

Note that this problem is caused by a bug in qml.math.is_independent, but so far only surfaces in the specific context of qnode_spectrum, which uses this function to check a Jacobian to be constant.

Description of the Change:
Add a warning to the docs that pure numpy parameters are not supported.
Raise an error if pure numpy parameters are present.
Smaller updates relating to code quality/linting.

Benefits:
Fixes #6593

Possible Drawbacks:
N/A

Related GitHub Issues:
Closes #6593
[sc-78453]

[github-actions]

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Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.46%. Comparing base (9c1292d) to head (ffd908d).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6622   +/-   ##
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  Coverage   99.46%   99.46%           
=======================================
  Files         454      454           
  Lines       42644    42649    +5     
=======================================
+ Hits        42415    42420    +5     
  Misses        229      229           

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[albi3ro]

Could we instead stop mapping "numpy" to "autograd" (by mapping "numpy" toNone) and raise an informative error if the interface is None`?

[albi3ro]

👍