Update spec_set to use lookup

[OughtPuts]

What was the end-user or developer problem that led to this PR?

Bundler::SpecSet already maintains a name-indexed lookup of specs (lookup), but #find_by_name_and_platform and the special rake handling in #sorted were still scanning @specs directly.

That meant these methods were not taking advantage of the existing indexed structure in the class, even though equivalent information was already available through lookup. The goal of this PR is to make those methods consistent with the rest of SpecSet by reusing the existing lookup instead of doing additional scans.

What is your fix for the problem, implemented in this PR?

This PR updates both call sites to use the existing name-indexed lookup:

• #find_by_name_and_platform now uses lookup[name]&.detect { ... } instead of scanning all of @specs
• #sorted now uses lookup["rake"]&.first instead of @specs.find { ... } for the special-case rake preloading behaviour.

The behaviour is intended to remain unchanged.

I also expanded the spec coverage in spec/bundler/spec_set_spec.rb to cover the important regression cases around the refactor:

• returning the matching platform-specific spec
• returning nil when the name is missing
• returning nil when the name exists but no spec is installable on the requested platform
• preserving the existing behaviour of returning the first matching installable spec
• preserving the special rake ordering behaviour in #to_a

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags