feat(types): define and export ServiceTier string literal (fix #3556)

[SparshGarg999]

Fixes #3556

Summary

Define and export ServiceTier as a string literal type (Literal["auto", "default", "flex", "scale", "priority", "fast"]) under openai.types and openai.types.shared.

This allows SDK consumers to import from openai.types import ServiceTier without hardcoding custom string literal unions that break across SDK updates.

How was this tested?

• Added unit test in tests/test_client.py verifying ServiceTier export and type literal values.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Mirror the nullable ServiceTier schema

The shared ServiceTier component is defined as the enum or null in api_reference/openapi.transformed.yml:56792-56812, and the generated response fields already expose service_tier as optional. Exporting the new public alias as only Literal[...] makes it narrower than the schema, so users annotating values read from responses (or nullable request values) still cannot use ServiceTier without adding their own | None. Please mirror the schema with Optional/None support.

AGENTS.md reference: AGENTS.md:L5-L6

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