fix(core): surface NamedError detail without doubling the tag

[albertdbio]

Issue for this PR

Closes #31414

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

NamedError's constructor called super(name), so every instance had Error.message === Error.name. Since Cause.pretty and Error.prototype.toString() render an error as ${name}: ${message}, this produced output like ProviderAuthError: ProviderAuthError and hid the actual data.message.

This sets Error.message to the human detail — data.message when the data carries a string message, otherwise empty — and keeps the tag on .name. Renderers then compose ProviderAuthError: <real message> once: no doubled tag, and the detail is no longer hidden.

util/log.ts's formatError returned the bare error.message, which after the above change would drop the tag. It now uses error.toString(), which composes ${name}: ${message} and omits the ": " separator when the message is empty — so errors whose data has no message field render as just the tag rather than an empty string.

How did you verify your code works?

Added packages/core/test/util-error.test.ts covering both paths: an error with a message renders as "Name: message" via both toString() and Cause.pretty; one without renders as "Name" (no trailing colon, no doubled tag). bun test passes locally and bun typecheck is clean across all packages.

Screenshots / recordings

N/A — not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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