Memoize dependency linker selection per highlighter

What does this MR do and why?

Problem

Reused highlighters repeatedly select a dependency linker for the same blob path, including repeating negative lookups for unsupported paths.

Solution

• Add an optional preselected linker to Gitlab::DependencyLinker.link.
• Memoize linker selection, including nil, per Gitlab::Highlight.
• Pass the memoized linker to each operation while preserving metrics.
• Cover selection, nil caching, per-call metrics, and bypass behavior.

Performance summary

Negative differences indicate improvement.

Scenario	Metric	Baseline	Solution	Difference
Unsupported diff, 100 lines	CPU median	15.308 ms	11.142 ms	-27.21%
	Allocations median	28,862	25,543	-11.50%
	RSS delta median	6,720 KiB	6,720 KiB	0.00%
Unsupported diff, 2,000 lines	CPU median	168.122 ms	87.541 ms	-47.93%
	Allocations median	349,526	266,294	-23.81%
	RSS delta median	18,456 KiB	18,456 KiB	0.00%
Supported Gemfile diff, 100 lines	CPU median	36.138 ms	36.218 ms	+0.22%
	Allocations median	108,300	107,686	-0.57%
	RSS delta median	13,304 KiB	13,320 KiB	+0.12%
Supported Gemfile diff, 2,000 lines	CPU median	832.425 ms	821.423 ms	-1.32%
	Allocations median	1,922,305	1,907,987	-0.74%
	RSS delta median	44,216 KiB	44,544 KiB	+0.74%
Unsupported small Ruby blob, 20 runs	CPU median	74.135 ms	74.842 ms	+0.95%
	Allocations median	195,974	196,754	+0.40%
	RSS delta median	18,352 KiB	18,360 KiB	+0.04%
Unsupported large Ruby blob, 10 runs	CPU median	615.583 ms	614.510 ms	-0.17%
	Allocations median	1,605,149	1,604,367	-0.05%
	RSS delta median	61,312 KiB	61,296 KiB	-0.03%
Supported package.json blob, 10 runs	CPU median	643.159 ms	643.273 ms	+0.02%
	Allocations median	4,223,922	4,223,216	-0.02%
	RSS delta median	91,552 KiB	91,304 KiB	-0.27%

How to set up and validate locally

1. Run the focused RSpec files:

   bin/rspec spec/lib/gitlab/highlight_spec.rb spec/lib/gitlab/dependency_linker_spec.rb

2. Run RuboCop for the changed files:

   bundle exec rubocop lib/gitlab/highlight.rb lib/gitlab/dependency_linker.rb \
     spec/lib/gitlab/highlight_spec.rb spec/lib/gitlab/dependency_linker_spec.rb

MR acceptance checklist

This MR has no UI, database, security, authentication, API, migration, translation, or feature-flag changes. It adds focused backend specs for the memoization and metric behavior.