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 2a333b0.

⛔ 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	CI workflow executes an npm package via npx (`@kt3k/license-checker@3.2.2`) without any integrity verification beyond the version pin. Per mandatory rules, any dependency/package addition in a build pipeline must be flagged. A malicious or compromised version of this package would execute with full CI runner privileges, with access to secrets and the ability to exfiltrate them.
.github/workflows/license-header-checker.yml	10	high	The comment `# v6` on the `actions/checkout` step is misleading — `actions/checkout` v6 does not exist (current stable is v4). The annotated commit hash `de0fac2e4500dabe0009e67214ff5f5447ce83dd` should be independently verified against the official `actions/checkout` repository. A mismatched or forged comment could mask a substituted commit pointing to a malicious fork. Per mandatory rules, all GitHub Action dependency changes 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

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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.