chore(sdk): migrate TypeScript 5 -> 6

[RBKunnela]

Summary

Migrate paybot-sdk to TypeScript 6. Covers compiler upgrade, resolving new errors/deprecations, and confirming the build + downstream consumers still type-check.

Scope

• Bump typescript to 6.x; update tsconfig for any changed defaults.
• Resolve all new type errors and deprecation warnings.
• Verify published type declarations remain consumable by downstream (paybot-mcp, Python parity build unaffected).

Acceptance criteria

• typescript on 6.x; npm run typecheck and build green.
• No new any/@ts-ignore introduced to silence migration errors (fix or document each).
• Emitted .d.ts declarations validated against a downstream consumer.
• Must include tests — existing suite green post-migration; add coverage for any behavior touched by an API/typing change.
• No secrets/env values printed.

Reference

Coupled with the paybot-mcp TS 5→6 migration (do them together or sequence sdk→mcp).

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DRAFT — not labeled night-shift. Toolchain migration; arm for the loop only when the operator approves (likely after B1 wire contract lands).