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 e203a91.

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

Path	Line	Severity	Description
.github/workflows/license-header-checker.yml	11	high	New npm package dependency introduced via npx in CI pipeline: @kt3k/license-checker@3.2.2. Per mandatory supply chain rule, all new package dependencies must be flagged regardless of apparent legitimacy. This package executes with full CI runner privileges and can exfiltrate secrets or modify source files.
.github/workflows/license-header-checker.yml	10	high	The actions/checkout action is pinned to commit hash de0fac2e4500dabe0009e67214ff5f5447ce83dd with a comment claiming '# v6', but actions/checkout v6 does not exist (latest is v4). A mismatch between the pinned hash and the version comment is a known supply chain attack pattern where a malicious commit hash is disguised with a plausible-looking version label. Maintainers must verify this hash resolves to the intended, trusted release.

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.

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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 78.53%. Comparing base (a8c55b9) to head (e203a91).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1470      +/-   ##
============================================
- Coverage     78.58%   78.53%   -0.05%     
  Complexity     1315     1315              
============================================
  Files           106      106              
  Lines          6046     6046              
  Branches        647      647              
============================================
- Hits           4751     4748       -3     
- Misses         1004     1008       +4     
+ Partials        291      290       -1     

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