feat: add web_link field to Result for browsable test links

[kamaxeon]

Summary

Add a web_link field to the Result dataclass in tmt/result.py that serializes
the browsable test source URL directly into results.yaml as web-link, so downstream
consumers (gluetools, kcidb) get a ready-to-use link instead of having to reconstruct it
from fmf-id.

• The field name is consistent with the existing Core.web_link() method and tmt test show output
• Populated via Test.web_link() in from_test_invocation()
• Returns None for local tests / shell discover (no URL available)
• Returns a browsable URL for tests from Git repos (GitLab, GitHub, Pagure, etc.)

Example output in results.yaml

- name: /tests/nvidia/driver-install
  result: pass
  fmf-id:
    url: https://gitlab.cee.redhat.com/gpu-accelerators/tests
    ref: main
    name: /tests/nvidia/driver-install
    path: .
  web-link: https://gitlab.cee.redhat.com/gpu-accelerators/tests/-/blob/main/tests/nvidia/driver-install/main.fmf
  duration: 00:05:23
  serial-number: 1

Ref: TFT-2683

Pull Request Checklist

• implement the feature
• write the documentation
• extend the test coverage
• update the specification
• adjust plugin docstring
• modify the json schema
• mention the version
• include a release note

[gemini-code-assist]

Code Review

Add a source_url field to the Result class, populate it using invocation.test.web_link(), and include unit tests for serialization. Strengthen the assertion in test_result_source_url_none_roundtrip by checking for the key's absence in the serialized output to avoid ambiguity.

[happz]

1. should ut be source-url, or taher web-link which is key used emitted by tmt test export? We should have one name for the property.
2. Update JSON schema in tmt/schemas/results.yaml, please.
3. Update the specification in spec/results.fmf, please.

[LecrisUT]

/packit build

[kamaxeon]

@LecrisUT ,

I just updated the MR request to fix a failing test on Fedora and CentOS.

[kamaxeon]

CI Status

The two remaining failures are not related to this PR:

1. testing-farm:fedora-44-x86_64:provision-image-mode-virtual — The /plans/provision/virtual/prepare schedule entry errored with Failure during Ansible playbook execution. This is a Testing Farm infrastructure issue during guest setup, not a test failure.

2. testing-farm:centos-stream-10-x86_64:full — The only failing test is /tests/multihost/complete (subresult: /Require-Test-Skipped-on-Client-2-with-error-return-code) under /plans/features/advanced. This is a pre-existing multihost test unrelated to the web_link changes in this PR (17 tests passed, 1 failed).

All *:full jobs that previously failed due to test_result_web_link_none_roundtrip are now passing.