chore: switch to SPDX license headers

[MikeGoldsmith]

Summary

• Replace the Apache 2.0 license header with the 2-line SPDX equivalent (# SPDX-License-Identifier: Apache-2.0) across all Python files
• Add scripts/check_license_header.py enforcement script and lint-license-header-check tox env to catch missing headers in CI
• Update scripts/license_header.txt template to use SPDX format
• Document the required header format in CONTRIBUTING.md

590 files had their existing Apache header replaced with the SPDX equivalent. During the migration, 85 files were found with missing headers and 15 files had non-standard header variants:

• 13 files used an older copyright format (Copyright 2020, OpenTelemetry Authors) — normalized to Copyright The OpenTelemetry Authors
• 2 files had Google LLC copyright with a different Apache URL format
• 1 file had a 6-space indent variant in the Apache URL
• 85 source/test/example files had no header at all

All now have the correct SPDX header. Third-party copyrights (Google LLC, Amazon) are preserved with SPDX on line 2.

Motivation

Other OTel SDKs and the Python core SDK have already adopted SPDX headers:

• Go: Already uses SPDX format
• Java: Already uses SPDX format
• Python core: chore: switch to SPDX license headers opentelemetry-python#5177

Closes #4521

Notes

• Generated files and empty __init__.py files are excluded from the check
• The check script accepts any # Copyright line (to support third-party copyrights like Google LLC, Amazon) followed by # SPDX-License-Identifier: Apache-2.0
• No runtime impact — Python strips comments during bytecode compilation

[tammy-baylis-swi]

Thanks!

This duplicates #4532 but also includes a workflow update. Not sure how you'd like to proceed, @MikeGoldsmith !