fix(inference-v2): simplify no-candidate error and return 503

[mcowger]

User description

Summary

• Simplify the target-exhaustion error message in the inference-v2 pi-ai executor to "No candidate targets available" (previously referenced pi_ai_provider/pi_ai_model_id internals and beta terminology).
• Return HTTP 503 instead of 400, matching the v1 path's behavior for this case.

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Description

• Simplify no-candidate error message to "No candidate targets available"

• Change HTTP status code from 400 to 503 for target exhaustion

• Update error code from no_beta_compatible_candidate to no_candidate_targets

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