CAMEL-23279: fix SamplingDefinition.description() NumberFormatException with property placeholders

[Croway]

Summary

• SamplingDefinition.description() calls TimeUtils.toDuration(samplePeriod) before property placeholders are resolved, causing NumberFormatException when using {{sample.period}}
• Changed to return the raw string value instead, consistent with how ThrottleDefinition.description() handles timePeriodMillis
• Added test for property placeholder usage with sample()

Test plan

• SamplingThrottlerTest#testSamplingWithPropertyPlaceholder — verifies route starts with {{sample.period}} placeholder
• All existing SamplingThrottlerTest tests pass (6/6)

[github-actions]

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