[OB-8109]: time format validation and fallback for demo logs

[akshaya-kumar-t]

No description provided.

[Copilot]

Pull Request Overview

This PR implements validation and fallback handling for time format strings in the demo_logs source's SampleFile format. The changes ensure robust timestamp formatting by validating the time format at configuration time and providing graceful degradation at runtime.

Key changes:

• Adds validation for time_format field to detect empty or invalid format strings during configuration
• Implements a fallback mechanism that uses RFC3339 format when timestamp formatting fails
• Adds comprehensive test coverage for both validation and fallback behavior

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[aayushoj]

Please address the copilot's comment on UT test case. LGTM.