camera_calibration_parsers: allow reading read-only INI files

[ktyang512]

Summary

• open calibration INI files with std::ifstream so reading does not require write permission
• add a cross-platform regression test for valid read-only calibration files
• restore the original file permissions or attributes during test cleanup

Fixes #397.

Testing

• reproduced the failure with the original std::fstream implementation
• camera_calibration_parsers-parse_ini: 7 tests passed, including the read-only regression under an ordinary Linux user
• independent read-only INI reproducer passed
• ROS Rolling package build and test in an Ubuntu Resolute container: 78 tests, 0 errors, 0 failures, 12 skipped
• ament_uncrustify and ament_cpplint passed for both changed files
• git diff --check passed

The read-only test performs a capability probe and skips when the test process can still open the file read/write, such as a privileged root container. The regression executed without skipping under the host ordinary user. Native Windows and macOS execution are left to CI.

Generative tool disclosure

Generated-by: OpenAI Codex (GPT-5)

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