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 3a285b8.

⛔ 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. Executing third-party packages directly in CI via npx introduces supply chain risk. Artifact authenticity cannot be verified from the diff alone — maintainers should confirm this is the intended package and version.
.github/workflows/license-header-checker.yml	10	high	New GitHub Actions dependency added: actions/checkout pinned to commit de0fac2e4500dabe0009e67214ff5f5447ce83dd (labeled v6). While commit-pinning is good practice, this is a new dependency introduction in a CI pipeline and must be verified against the official actions/checkout repository to confirm the commit corresponds to the stated v6 tag.

The table above displays the top 10 most important findings.

Total: 2 | Critical: 0 | High: 2 | 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.