Full-stack: Make "Server url" validation conditions consistent across Fleet, update Web Address form validation and submission logic per Fleet best practices (frontend/docs/patterns.md)

[jacobshandling]

For #27454

Consider Fleet web URL to be valid if it:

• (Front end and back end): uses “https://” or “http://” scheme
  and
• (Front end) accepts only valid or "localhost" hosts (e.g., "a.b.cc" or "localhost", but not "a.b")
• (Back end) accepts any host (e.g., "localhost", "a.b.cc", or even "a.b")

Setup flow UI URL validation:

Org settings UI URL validation:

Server URL validation:

Invalid Fleet server URL in DB error:

• Changes file added for user-visible changes in changes/,
• Added/updated automated tests
• A detailed QA plan exists on the associated ticket (if it isn't there, work with the product group's QA engineer to add it)
• Manual QA for all new/changed functionality

[codecov]

Codecov Report

Attention: Patch coverage is 60.86957% with 9 lines in your changes missing coverage. Please review.

Project coverage is 63.76%. Comparing base (67e5541) to head (96be402).
Report is 49 commits behind head on main.

Files with missing lines	Patch %	Lines
server/service/validation_setup.go	45.45%	3 Missing and 3 partials ⚠️
server/service/appconfig.go	57.14%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #27455      +/-   ##
==========================================
- Coverage   63.78%   63.76%   -0.03%     
==========================================
  Files        1742     1746       +4     
  Lines      165562   165854     +292     
  Branches     4534     4536       +2     
==========================================
+ Hits       105605   105756     +151     
- Misses      51731    51836     +105     
- Partials     8226     8262      +36     

Flag	Coverage Δ
backend	64.57% <50.00%> (-0.03%)	⬇️
frontend	54.28% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[jacobshandling]

Okay, so: PR currently has everywhere on front and backend following the same rules, and validation + submission logic on the Web Address org settings form up-to-date with Fleet's best practices

Before these updates, there were different conditions enforced in different places. The conditions for validation are now, everywhere:

• must have “https://” or “http://” scheme with host (e.g. "http://a.b.cc")
  OR
• Must begin with “localhost” (e.g. "localhost:8080", "http://localhost:8080", "https://localhost:8080" are all okay)

If we want to refine these conditions, we can.

Significantly - this adds the requirement of a valid scheme to the Web Address org settings form, unless the URL begins with "localhost". An alternative is to not require a scheme more generally, and apply that to all 4 places outlined in this PR description. Another alternative is to only require a scheme. This could interfere with any users who have saved Fleet server URLs without schemes, which @sgress454 and I confirmed is, while not a great practice and pretty unlikely, a possible scenario.

[sgress454]

LGTM, may as well hold off merging until after meeting w/ product later today.

[sgress454]

This still allows "localhost.com" etc., which would then be blocked by the back end. I don't think it's worth the effort to handle that here before we talk to the product team about this though.

[jacobshandling]

This is because the frontend actually does validate both the scheme and a valid hostname