<CI>(workflow): restore Codecov unit-test coverage upload

[kyonRay]

What

Restores Codecov coverage reports on pull requests.

Why

Codecov stopped reporting coverage after commit 9837e924 (#941), which deleted the build-centos job. That job carried the only coverage-upload step in the workflow:

- name: upload unittest coverage
  run: curl -LO https://codecov.io/bash && /bin/bash ./bash

Since then the remaining build matrix job runs tests but never uploads a JaCoCo report, so Codecov has no data and posts no coverage report/comment on any PR. .codecov.yml and the jacoco plugin (jacocoTestReport, xml.enabled = true) are still configured — only the upload was missing.

Change

Adds a dedicated coverage job that:

• runs the unit tests and generates the JaCoCo XML report via gradlew test jacocoTestReport;
• uploads build/reports/jacoco/test/jacocoTestReport.xml to Codecov with the existing CODECOV_TOKEN secret (codecov/codecov-action@v4).

Notes:

• Unit-test scope is intentional and matches .codecov.yml, which already ignores src/integration-test/** and src/integration-wasm-test/**. So this needs no live FISCO BCOS node and is fast/deterministic.
• The job is continue-on-error: true, so it never blocks a PR.
• No untrusted input is used in any run: step (only the CODECOV_TOKEN secret), so there is no workflow-injection surface.

🤖 Generated with Claude Code

[Copilot]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (release-3.9.0@861d8f0). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-3.9.0     #958   +/-   ##
================================================
  Coverage                 ?   52.08%           
  Complexity               ?     4040           
================================================
  Files                    ?      430           
  Lines                    ?    17592           
  Branches                 ?     1962           
================================================
  Hits                     ?     9163           
  Misses                   ?     7699           
  Partials                 ?      730           

Flag	Coverage Δ
unittest	52.08% <ø> (?)

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[sonarqubecloud]

Quality Gate failed

Failed conditions
C Security Rating on New Code (required ≥ A)

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