backend: (x86) report unsupported vector sizes without the raw KeyError

[manishpaulish]

Split out of #6360 as requested, with a dedicated test.

_register_type_for_vector_type raises DiagnosticException from inside except KeyError, which chains the dict lookup onto the traceback. A vector too wide for the target therefore reports:

KeyError: 512

During handling of the above exception, another exception occurred:
...
DiagnosticException: The vector size (512 bits) and target architecture `avx2` are inconsistent.

The diagnostic underneath is correct, but KeyError: 512 is the first thing you see and it points at a dict rather than at the problem.

Two changes:

• suppress the chain with raise ... from None
• name the sizes the target does support, since "inconsistent" alone does not tell you whether the vector is too wide, too narrow, or an odd width

DiagnosticException: The vector size (512 bits) and target architecture `avx2`
are inconsistent. Supported vector sizes are [128, 256].

The test covers all three targets and asserts the KeyError is not chained, via __cause__ and __suppress_context__, so a future refactor that drops the from None fails rather than quietly regressing the message.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.93%. Comparing base (b642df9) to head (f90b17d).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6361   +/-   ##
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  Coverage   86.93%   86.93%           
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  Files         439      439           
  Lines       65902    65919   +17     
  Branches     7490     7490           
=======================================
+ Hits        57289    57304   +15     
- Misses       7036     7038    +2     
  Partials     1577     1577           

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