Expose message_channel_ids on opal_resource via overlay entity mapping

[claude]

Requested via Slack thread

Linear: COR-5621

Description of the change

Addresses #197.

Before: opal_resource has no message_channel_ids attribute, even though the API supports GET/PUT /resources/{resource_id}/message-channels — so audit-message channels on resources can't be managed in Terraform (only read via the opal_resource_message_channel_list data source).

After: opal_resource gains a settable message_channel_ids attribute (plus computed message_channels), matching what opal_group already offers.

Approach: overlay-only entity-mapping change in terraform_overlay.yaml, mirroring the existing group pattern — the GET is merged into the resource entity as Resource#read#2 (response allOf-wrapped with x-speakeasy-wrapped-attribute: message_channels and x-speakeasy-param-suppress-computed-diff), and the PUT becomes Resource#create#4 + Resource#update#3 with the 200 response ignored. The standalone opal_resource_message_channel_list data source mapping is kept alongside via a multi-entity operation list (same precedent as Group#delete + AccessRule#delete).

How / notes for maintainers: internal/sdk/, openapi.yaml, and docs/ are Speakeasy-generated, so this PR intentionally touches only terraform_overlay.yaml — please run the sdk_generation workflow on this branch to regenerate before un-drafting (same flow as #200). On regen, please double-check: (1) opal_resource is widely used, so state-upgrader/import impact of the new attributes should be verified (internal/stateupgraders); (2) the new x-speakeasy-name-override: id on the GET parameter may rename the opal_resource_message_channel_list data source's resource_id argument to id (as happened for opal_resource_visibility) — if that rename is unwanted, the override can be dropped, but the entity merge may then need the param mapped differently.

Checklist

• I performed a self-review of my code
• I manually tested my code change (please list details in description) — not possible locally; requires the Speakeasy regen workflow
• I added unit tests — n/a until regen
• I updated the changelog — pending regen
• I updated the public facing docs — docs are generated on regen

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