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

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[github-actions]

PR Code Analyzer ❗

AI-powered 'Code-Diff-Analyzer' found issues on commit c65ee2b.

⛔ 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	Supply chain risk: introduces `npx @kt3k/license-checker@3.2.2` executed directly in CI. Per mandatory flagging rule, any new package dependency must be flagged regardless of apparent legitimacy. An npm package run via npx in CI has full access to the runner environment including secrets.
.github/workflows/license-header-checker.yml	10	high	Suspicious version annotation: `actions/checkout` is pinned to commit `de0fac2e4500dabe0009e67214ff5f5447ce83dd` with comment `# v6`, but `actions/checkout` has no v6 release (latest is v4). The comment is unverifiable against any real release tag, making it impossible to confirm this hash corresponds to the legitimate action. This pattern is consistent with substituting a malicious fork while using a plausible-looking comment.

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.