feat(logs): add storage tier support for Flex logs

[robhughadams]

What does this PR do?

Adds --storage flag to all log search commands (search, list, query, aggregate) enabling users to query specific Datadog log storage tiers: indexes (standard), online-archives (long-term), and flex (cost-optimized).

Changes:

• Added logsStorage flag variable in cmd/logs_simple.go:548
• Implemented validateAndConvertStorageTier() helper function (cmd/logs_simple.go:685-705)
• Added --storage flag to logs search, list, query, aggregate commands
• Modified runLogsSearch(), runLogsList(), runLogsQuery(), runLogsAggregate() to validate and apply storage tier
• Added comprehensive test coverage with 17 test cases in cmd/logs_simple_test.go
• Updated docs/EXAMPLES.md with storage tier usage examples
• Updated docs/COMMANDS.md with Flex logs example

Features:

• Validates storage tier before API calls for fast failure
• Case-insensitive input handling (FLEX, flex, Flex all work)
• Whitespace trimming for user convenience
• Helpful error messages showing valid options
• Backward compatible: omitting --storage searches all tiers (default behavior)

Testing:

• TestValidateAndConvertStorageTier: 9 test cases covering validation logic
• TestRunLogsSearchWithStorageTier: 5 test cases for search command
• TestRunLogsQueryWithStorageTier: 2 test cases for query command
• TestRunLogsAggregateWithStorageTier: 1 test case for aggregate command
• All tests pass with existing test suite
• Full cmd package coverage maintained

Motivation

I wanted to be able to use the command line to search flex tier logs as that's not supported at the moment.

Checklist

• The code change follows the project conventions (see CONTRIBUTING.md)
• Tests have been added/updated (if applicable)
• Documentation has been updated (if applicable)
• All CI checks pass
• Code coverage is maintained or improved

Related Issues

I haven't created an issue for this - let me know if you'd like that before getting the PR.