fix(privacy): Filter search case sensitivity on iOS

[StephenHeaps]

• We were lowercasing the user-entered search text, but then performing a case-sensitive search on the title & description of the filters.

Resolves brave/brave-browser#41168

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run presubmit wiki, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

1. Open app Settings -> Brave Shields & Privacy -> Content Filtering
2. Scroll down slightly to reveal search bar
3. Enter You as search text
4. Verify all filters with You are visible
5. Change search text to YoUtUbE
6. Verify all filters with YouTube are visible
7. Change search text to easylist
8. Verify all filters with EasyList are visible

You	YoUtUbE	easylist

[Uni-verse]

Verified on iPhone 12 running iOS 18 using version 1.72 (49)

Test Plan

1. Open app Settings -> Brave Shields & Privacy -> Content Filtering
2. Scroll down slightly to reveal search bar
3. Enter You as search text
4. Verify all filters with You are visible
5. Change search text to YoUtUbE
6. Verify all filters with YouTube are visible
7. Change search text to easylist
8. Verify all filters with EasyList are visible

• Ensured that filter search is not case sensitive using search bar.
• Ensured that External Filter Lists as well as Default Filter lists are searchable.
• Ensured that both filter title and content is searchable.

"youtube"	"YoUtUbE"	"You"	"Ho" (searching title + content)	Returning both lists