Add inactivity_timeout support to Fleet agent policy resource with duration string type and version validation

[Copilot]

• Add initial inactivity_timeout support to Fleet agent policy resource
• Add version validation for inactivity_timeout (8.7.0+)
• Add comprehensive test coverage for inactivity_timeout functionality
• Fix field handling to prevent sending pointer to zero values for unknown attributes
• Convert inactivity_timeout to user-friendly duration string format
• Apply code review suggestions for duration conversion and test helper simplification

Changes Made

Duration String Implementation

• Schema: Added inactivity_timeout as optional, computed String attribute using customtypes.DurationType
• User Experience: Supports intuitive duration strings like "30s", "2m", "1h" instead of raw seconds
• API Integration: Automatically converts duration strings to seconds for Fleet API and converts API responses back to duration strings
• Documentation: Updated to reflect string duration format with examples

Core Features

• Model: Added field to handle the new attribute in API requests/responses with proper duration conversion
• API Integration: Updated create and update operations to pass timeout to Fleet API with duration parsing
• Version Validation: Added minimum version check (8.7.0+) with proper error handling
• Field Handling: Fixed issue where ValueFloat32Pointer/ValueBoolPointer would send zero values for unknown attributes
• Testing: Added comprehensive test coverage including explicit timeout configuration and version validation

Code Review Updates

• Duration Conversion: Improved duration conversion in models.go to use time.Duration with proper truncation instead of string formatting
• Test Helpers: Simplified test helper functions by removing hardcoded parameters where only single values are used

Field Handling Fix

The previous implementation would always set SupportsAgentless and InactivityTimeout fields in the API request body using ValueBoolPointer() and ValueFloat32Pointer(), which return pointers to zero values (false/0) when the Terraform values are unknown. This could send unintended values to the API.

The fix moves the body declaration above version checks and only sets these fields conditionally when their values are actually known, preventing zero-value pointers from being sent to the API for unknown attributes.

Duration Type Benefits

Users can now specify timeouts using intuitive duration strings:

• inactivity_timeout = "30s" (30 seconds)
• inactivity_timeout = "2m" (2 minutes)
• inactivity_timeout = "1h" (1 hour)

The implementation maintains full compatibility with the Fleet API while providing a much more user-friendly interface that follows Go duration string conventions.

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[dimuon]

Besides one nit-pick question, it looks good 👍 Also it makes sense to redefine the timeout as a string duration.