Validate client credential type

[cryptoryda]

Reject unsupported Tempo client credential types

Summary

• validate Config.CredentialType in chargeclient.New
• preserve the empty value as the existing transaction default
• add regression coverage for an unsupported value

Why

Unknown credential types previously survived construction and could be
silently serialized as transaction credentials. Failing at construction makes
configuration mistakes explicit and prevents unintended credential behavior.

Testing

go test ./pkg/tempo/client
go test ./...
go vet ./...

[brendanjryan]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Swish!

Reviewed commit: fe0e95e729

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