Add support for custom error message and link in codec server for namespaces. Closes #408

[billrich2001]

What was changed

Add optional fields to codec_server for custom error message and link

Why?

Spurred by customer ticket; Match functionality with UI.

Checklist

1. Closes [Feature Request] codec_server custom error message and link #408

2. How was this tested:
   Added unit tests, and ran against existing tests and CI

3. Any docs updates needed?
   Auto generated with changes

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Note

Medium Risk
Moderate risk because it extends the codec_server schema and request/response mapping for namespaces, which can affect plan/state and API payloads, but changes are additive and covered by new unit/acceptance tests.

Overview
Adds optional codec_server.custom_error_message and codec_server.custom_error_link to the temporalcloud_namespace resource and exposes the same fields as computed outputs on the temporalcloud_namespace(s) data sources.

Updates the namespace spec/state mapping to read/write these fields to the Cloud API (CustomErrorMessage.Default.{Message,Link}), and expands test coverage (acceptance + unit) to validate null vs set behaviors and import stability. Docs are regenerated to reflect the new attributes.

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[anekkanti]

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[anekkanti]

@billrich2001 Thank you for the PR.