refactor: share SDK client context

[victor0602]

What changed

• Add a shared SDK client context that resolves configuration once.
• Reuse the same context and config across all MiniMaxSDK child modules.
• Preserve direct construction of individual SDK modules with MiniMaxSDKOptions.

Why

Constructing MiniMaxSDK previously rebuilt the same configuration independently for every child SDK.

Impact

All child SDKs now observe one consistent configuration snapshot without breaking standalone SDK module usage.

Checks

• bun test — 451 passed
• bun run typecheck
• bun run lint — no errors; one pre-existing test warning
• git diff --check

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