Standalone Activities: add activity_ prefix to id_conflict/reuse_policy

[yuandrew]

What changed?

Describe what has changed in this PR.
Add activity_ prefix to existing fields

Why?

Tell your future self why have you made these changes.
temporalio/api#707

How did you test it?

• built
• run locally and tested manually
• covered by existing tests
• added new unit test(s)
• added new functional test(s)

Potential risks

Any change is risky. Identify all risks you are aware of. If none, remove this section.
Should be low/none, standalone activities not yet released

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Note

Low Risk
Mechanical field rename/wiring change limited to standalone activity start/validation; main risk is misrouting defaults if callers still rely on deprecated unprefixed fields.

Overview
Standalone activity start now reads ID reuse/conflict policy from the new ActivityIdReusePolicy/ActivityIdConflictPolicy request fields, including updated invalid-argument errors.

Validation/normalization likewise defaults the new fields when unspecified, ensuring standalone activities honor the activity_-prefixed API fields consistently.

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[yuandrew]

scrapping effort, not worth the pain of coordinating the correction across SDK/server