* feat: support sampling with tools
* address sampling tools review feedback
* tighten sampling tools spec compliance
* drop SamplingStopReason enum in favor of open string
The spec leaves stopReason open for provider-specific values, so upcasting
the four known ones to enum cases makes every future enum addition a silent
BC break for string comparisons.
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Co-authored-by: Christopher Hertel <mail@christopher-hertel.de>
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