Fix empty modified-lines REST API by making exported beans public

[jeppeman]

The modified-lines coverage REST API (.../coverage/modified/api/json) returns a response
with no files array on recent Jenkins cores, even though the coverage data is computed
and stored correctly:

{"_class":"io.jenkins.plugins.coverage.metrics.restapi.ModifiedLinesCoverageApi"}

Root cause: the classes backing this endpoint — ModifiedLinesCoverageApi,
FileWithModifiedLines, ModifiedLinesBlock, and the LineCoverageType enum — and their
@Exported getters are package-private. Recent Stapler releases only export @Exported
members that are public, so ModelBuilder finds zero exported properties and the
files property is silently dropped. Older Stapler reflected non-public members, which is
why this regressed on a Jenkins core upgrade without any change to the plugin:

Jenkins core	Bundled Stapler	files in response
2.511	1983.v93c53e94b_c04	present ✅
2.555.3	2076.v1b_ac12445eb_e	missing ❌

The visibility was reduced in cc8cdbd7 ("visibility changes in restapi package"); the
defect stayed latent until Stapler's behavior tightened.

Fix: make the four types and their @Exported accessors public.

Fixes #785

Testing done

Added ModifiedLinesCoverageApiTest#shouldExportModifiedLinesAsJson, which serializes the
bean through the same Stapler export path the endpoint uses (ModelBuilder +
Flavor.JSON) and asserts a non-empty files array. Verified it fails on the
package-private code and passes with the visibility fix:

• Unpatched (package-private): Different value found in node "files", expected: <array> but was: <missing>
• Patched (public): all 4 tests in the class pass

The existing integration test that exercises this endpoint
(GitForensicsITest#verifyModifiedLinesCoverageApi) is currently @Disabled, so this
path had no active coverage — which is why the regression wasn't caught.

Also manually reproduced end-to-end: on Jenkins 2.555.3 with coverage
3.3278.va_04d3e352354, a build whose stored coverage data contains modified lines
(confirmed via filterByModifiedLines().getAllFileNodes()) returned an empty response
from /coverage/modified/api/json; after installing a locally built plugin with this fix,
the same build returned the fully populated files array.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and
  not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is
  fixed

[github-actions]

☀️   Quality Monitor

Tests

     Unit Tests: ✅ successful — 221 passed $\color{green}{\textsf{(+1)}}$
   🚀   Integration Tests: ✅ successful — 157 passed $\textsf{(±0)}$, 7 skipped $\textsf{(±0)}$
   ⛔   Architecture Tests: ✅ successful — 15 passed $\textsf{(±0)}$

Coverage for New Code

   〰️   Line Coverage: 100.00% — perfect 🎉
   ➰   Branch Coverage: 100.00% — perfect 🎉

Coverage for Whole Project

   〰️   Line Coverage: 77.00% $\textsf{(±0)}$ — 714 missed lines
   ➰   Branch Coverage: 67.38% $\textsf{(±0)}$ — 288 missed branches

Style

     CheckStyle: No warnings $\textsf{(±0)}$
     PMD: No warnings $\textsf{(±0)}$
   ☕   Java Compiler: No warnings $\textsf{(±0)}$

Bugs

     SpotBugs: No bugs $\textsf{(±0)}$
   🐛   Error Prone: No bugs $\textsf{(±0)}$

Vulnerabilities

   🛡️   OWASP Dependency Check: 255 vulnerabilities $\color{green}{\textsf{(+255)}}$ — error: 17, high: 78, normal: 151, low: 9

Software Metrics

   🌀   Cyclomatic Complexity: 1039 (total)
   💭   Cognitive Complexity: 569 (total)
   ➿   N-Path Complexity: 1394 (total)
   📏   Lines of Code: 12175 (total)
   📝   Non Commenting Source Statements: 4106 (total)
   🔗   Class Cohesion: 100.00% (maximum)
   ⚖️   Weight of Class: 100.00% (maximum)

📌 Reference Results

Delta reports computed against the reference results of 0181950 in workflow run 29743580601.

🚦 Quality Gates

Overall Status: ✅ SUCCESS

✅ Passed Gates

• ✅ Overall Tests Success Rate: 100.00 >= 100.00
• ✅ Line Coverage in New Code: 100.00 >= 90.00
• ✅ Branch Coverage in New Code: 100.00 >= 90.00
• ✅ Potential Bugs in Whole Project: 0.00 <= 0.00
• ✅ Style Violation in Whole Project: 0.00 <= 0.00

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Created by Quality Monitor v4.15.0 (#82d77af). More details are shown in the GitHub Checks Result.

[uhafner]

Thanks!