Add SPDX license header checker and missing headers

[shreyah963]

Description

Add SPDX license header validation to PR/push workflows using @kt3k/license-checker.

Changes:

• Add .github/workflows/license-header-checker.yml
• Add .licenserc.json
• Add missing SPDX-License-Identifier: Apache-2.0 headers to source files

Related Issues

#6445

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

PR Code Analyzer ❗

AI-powered 'Code-Diff-Analyzer' found issues on commit 8fb7425.

⛔ Hard block: Issues at High severity or above will block this PR from merging.

Path	Line	Severity	Description
.github/workflows/license-header-checker.yml	12	high	New npm package introduced via npx: @kt3k/license-checker@3.2.2. Per mandatory supply chain policy, all new package dependencies must be flagged for maintainer verification regardless of apparent legitimacy.
.github/workflows/license-header-checker.yml	10	high	New GitHub Actions dependency added: actions/checkout@de0fac2. Per mandatory supply chain policy, all new build/CI plugin and action dependencies must be flagged for maintainer verification.

The table above displays the top 10 most important findings.

Total: 2 | Critical: 0 | High: 2 | Medium: 0 | Low: 0

---

Pull Requests Author(s): Please update your Pull Request according to the report above.

Repository Maintainer(s): You can bypass diff analyzer by adding label skip-diff-analyzer after reviewing the changes carefully, then re-run failed actions. To re-enable the analyzer, remove the label, then re-run all actions.

---

⚠️ Note: The Code-Diff-Analyzer helps protect against potentially harmful code patterns. Please ensure you have thoroughly reviewed the changes beforehand.

Thanks.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.55%. Comparing base (3a69aab) to head (8fb7425).
⚠️ Report is 119 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #999      +/-   ##
============================================
- Coverage     73.97%   70.55%   -3.42%     
- Complexity      916     1149     +233     
============================================
  Files           135      175      +40     
  Lines          6102     7971    +1869     
  Branches        753      956     +203     
============================================
+ Hits           4514     5624    +1110     
- Misses         1253     1948     +695     
- Partials        335      399      +64     

☔ View full report in Codecov by Harness.
📢 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.