feat(keyring-api): add assertCreateAccountOptionIsSupported helper (for createAccounts)

[ccharly]

Helper to guide TS and narrow down CreateAccountOptions.type to a specific set of values.

This way, a createAccounts implementation can assert this and work with specific types rather than having to deal with a default branch/case for the unsupported types.

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Note

Low Risk
Adds a small runtime assertion helper plus tests; no changes to keyring execution paths beyond optional use of the helper.

Overview
Adds a new assertCreateAccountOptionIsSupported helper in the v2 create-account API to let createAccounts implementations reject unsupported CreateAccountOptions.type values and narrow the option type in TypeScript.

Includes comprehensive unit tests covering supported/unsupported type behavior and compile-time narrowing expectations, and documents the addition in the keyring-api changelog.

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

[danroc]

Maybe we should assert the type of options instead of options.type.

[ccharly]

Yup, I agree, this would synergies pretty well with createAccounts params itself. I'll change this, thanks!