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 627f2d6.

⛔ 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 third-party package introduced via npx: @kt3k/license-checker@3.2.2. Per mandatory supply chain policy, all new package dependencies must be flagged regardless of apparent legitimacy. This package executes in CI on every push and pull_request, giving it access to the runner environment, secrets, and checked-out code. Maintainers should verify the package's authenticity, ownership, and postinstall/runtime behavior before merging.

The table above displays the top 10 most important findings.

Total: 1 | Critical: 0 | High: 1 | Medium: 0 | Low: 0

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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.

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⚠️ 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 70.41%. Comparing base (9cb092f) to head (627f2d6).
⚠️ Report is 72 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #976      +/-   ##
============================================
- Coverage     70.88%   70.41%   -0.47%     
- Complexity      421      525     +104     
============================================
  Files            49       56       +7     
  Lines          3125     3664     +539     
  Branches        194      244      +50     
============================================
+ Hits           2215     2580     +365     
- Misses          785      920     +135     
- Partials        125      164      +39     

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