Refactor JWT::JWK::Set#initialize

[anakinj]

Description

Pure refactor of JWT::JWK::Set#initialize, no behaviour change. Groundwork for #744.

The two branches that build keys from hashes had drifted apart: the JWKS document form (Hash) skips a key whose kty is unsupported, per RFC 7517 §5, while the array form raises. Both were each_with_object/map blocks nested in the middle of a case, which makes the difference between them easy to miss — and that is how the gap #744 reports came about.

Each branch now calls a named method, so the difference is stated rather than buried:

when Hash  then build_supported_keys(jwks.transform_keys(&:to_sym)[:keys], options)
when Array then build_keys(jwks, options)

Side effects of the extraction:

• initialize fits the metrics limits, so the # rubocop:disable Metrics/CyclomaticComplexity it has carried is gone.
• jwks = jwks.transform_keys(&:to_sym) no longer reassigns the subject of the case from inside one of its own branches; jwks ||= {} becomes an explicit when nil then [].

On #744

#744 currently fails RuboCop, because copying the rescue block into the array branch pushes initialize to Metrics/MethodLength [22/18] and Metrics/PerceivedComplexity [9/8]. On top of this refactor that PR becomes a one-line change — point the Array branch at build_supported_keys and delete build_keys — with its spec unchanged.

Verification

• Full suite green, RuboCop clean on the file with no exemptions.
• Behaviour compared against main across 22 input shapes (nil / no args / empty hash, symbol and string keys, unsupported kty in first, last and only position, keys: nil, keys: given a bare hash, a bare JWK hash, arrays of hashes and of JWK objects, a single JWK object, construction from a Set, copy independence, bad types, options pass-through, merge). Output is identical, including the array-with-unsupported-kty cases still raising JWT::UnsupportedKeyType.

Checklist

Before the PR can be merged be sure the following are checked:

• There are tests for the fix or feature added/changed
• A description of the changes and a reference to the PR has been added to CHANGELOG.md. More details in the CONTRIBUTING.md