Add ownership checks for storage directories

[rads-1996]

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds hardening for local storage by verifying directory ownership on Unix-like systems and using atomic file creation to mitigate race-condition style attacks.

Changes:

• Switch temp-file writes to os.open(..., O_CREAT | O_EXCL, ...) to prevent overwrite/race conditions when creating .tmp files.
• Add Unix directory ownership validation (must be owned by current uid or root) before enabling local storage.
• Expand unit tests to cover ownership outcomes and attacker scenarios (pre-created dirs, symlinks, pre-created temp files).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
sdk/monitor/azure-monitor-opentelemetry-exporter/tests/test_storage.py	Updates existing tests for os.open and adds new tests for directory ownership and attack scenarios.
sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/_storage.py	Implements atomic .tmp creation via O_EXCL and adds Unix ownership checks using lstat/getuid.
sdk/monitor/azure-monitor-opentelemetry-exporter/CHANGELOG.md	Documents the new ownership-check behavior in the unreleased changelog.