Add --flow-tag flag to list trails command

[jumboduck]

This change adds the flow-tag flag to the list trails command.

[Copilot]

Pull request overview

This PR adds a --flow-tag flag to the list trails command, enabling users to filter trails by flow tags. The change allows listing trails across all flows that are tagged with a specific key-value pair.

Changes:

• Added --flow-tag flag to filter trails by flow tags in key=value format
• Implemented validation to ensure the flow-tag format is correct
• Added comprehensive test coverage for the new flag functionality

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
cmd/kosli/listTrails.go	Added flow-tag flag, validation logic, and URL parameter construction
cmd/kosli/root.go	Added flag description constant for the new flow-tag flag
cmd/kosli/listTrails_test.go	Added test cases for flow-tag filtering including valid tags, non-existing tags, and invalid format
cmd/kosli/testHelpers.go	Added TagFlow helper function to support testing flow tagging
cmd/kosli/list.go	Removed duplicate validateForListTrails function

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