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

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

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 4b8858c.

⛔ 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 introduces an npm package dependency (`@kt3k/license-checker@3.2.2`) executed via `npx` in the build environment. Per mandatory supply chain rules, all dependency introductions must be flagged regardless of apparent legitimacy — maintainers should verify the package on npm, confirm the publisher identity, and review the package's postinstall/execution 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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