feat: add user-template write endpoints to TemplateClient

[scottlovegrove]

Summary

Stacks on top of #603. Adds the full /templates/user/* surface to the SDK:

• createUserTemplate — POST /templates/user
• updateUserTemplate(id, args) — PUT /templates/user/{id}
• deleteUserTemplate(id) — DELETE /templates/user/{id}
• previewUserTemplateFromFile — POST /templates/user/preview_from_file (multipart)
• createUserTemplateFromFile — POST /templates/user/import (multipart, accepts uploadedFileName from a prior preview)

updateUserTemplate / deleteUserTemplate validate templateId is non-empty before any network call and URL-encode the path segment.

Test plan

• npm run check (oxlint + oxfmt) passes
• npm test — 602 tests pass (9 new)
• Tests cover JSON payloads (camelCase → snake_case conversion), URL-encoding of the path segment, empty-templateId rejection, multipart additional fields, and uploadedFileName pass-through

🤖 Generated with Claude Code

[doistbot]

This PR successfully expands the SDK's TemplateClient by introducing the full suite of write and file-import endpoints for user templates. The thorough test coverage and careful Zod schema modeling provide a solid foundation for these new capabilities. There are just a few remaining details to refine, such as addressing a bug that prevents the file upload optimization from working, strengthening types with ColorKey and enums, centralizing the URL-encoding logic, and removing some duplication in the test mocks and argument interfaces.

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