docs: show usage budget checks

[EfeDurmaz16]

Summary

• Add a streaming docs example that checks stream.usage.total_tokens against an application budget.
• Clarify in the core streams guide that usage can drive app-level token budgets, tenant quotas, and workflow limits.
• Call out that missing provider usage should be handled as an explicit application policy decision.

Why
This keeps rate/spend controls at the application boundary while showing where the SDK exposes provider-normalized usage.

Verification

• pnpm --dir docs/ai-python install
• pnpm --dir docs/ai-python build
• git diff --check

Notes

• uv run ruff format --check ... does not support MDX files and fails to parse frontmatter, so I used the docs build as the relevant validation.

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