feat: add default_org_member_roles to coderd_organization

[Emyrk]

Refs PLAT-217, depends on coder/coder#25994.

Adds a default_org_member_roles set attribute to coderd_organization so callers can override the deployment-wide default member roles per organization.

• New attribute is Optional + Computed; omitting it leaves the deployment defaults in place.
• Create flow follows the workspace_sharing precedent: post-create UpdateOrganization PATCH when the user provides a value, since CreateOrganization doesn't accept the field.
• Update flow plumbs the value through the existing UpdateOrganization call as a *[]string so an unset attribute does not overwrite server state.
• Bumps coder/coder to a SHA on the gateway-accounts stack and reacts to feat!: patchTemplateMeta to use optional fields coder#24984, which migrated UpdateTemplateMeta fields from values to optional pointers.

Agent context

• internal/provider/organization_resource.go: new field on the resource model and schema, plus a defaultOrgMemberRolesValueFromAPI helper that maps a nil slice to an empty set so the attribute always has a known value.
• internal/provider/organization_resource_test.go: new acceptance step under enableExperimentalSteps that sets the field to ["organization-member", "organization-template-admin"] and asserts state. The happy-path test now enables the minimum-implicit-member experiment alongside workspace-sharing since the server gates non-default values behind it.
• internal/provider/template_resource.go and internal/provider/template_data_source_test.go: react to feat!: patchTemplateMeta to use optional fields coder#24984's pointer migration on UpdateTemplateMeta. Mechanical conversions: .ValueString() becomes .ValueStringPointer() etc., and bool literals get wrapped with ptr.Ref(...).
• docs/resources/organization.md: regenerated via make gen.

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_{Coder Agents on behalf of @Emyrk.}

[linear-code]

PLAT-217