Allow hosts to customize wait_for_environment descriptions

[copyberry]

Allow hosts to customize wait_for_environment descriptions

What changed

• Add WaitForEnvironmentToolConfig as thread extension data for overriding the
  model-visible tool and environment_id descriptions.
• Preserve the default descriptions when no override is provided or when the
  configured descriptions exceed the input or serialized tool-spec limits.
• Keep wait_for_environment availability gated by the deferred executor
  feature independently of whether a host override is present.

Testing

• Cover default, custom, oversized, and feature-disabled tool configurations.
• Verify the custom descriptions in the deferred-environment integration flow.

[github-actions]

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