feat(template): add Template publish and unpublish methods [DX-903] [DX-904]

[Michael Pham (michaelphamcf)]

Summary

Adds publish and unpublish methods for the Template entity in the CMA SDK, completing DX-903 and DX-904.

Description

• publish: PUT /spaces/{spaceId}/environments/{environmentId}/templates/{templateId}/published — sends X-Contentful-Version header, returns updated TemplateProps
• unpublish: DELETE /spaces/{spaceId}/environments/{environmentId}/templates/{templateId}/published — sends X-Contentful-Version header, returns updated TemplateProps

Changes across the standard layers:

• lib/common-types.ts — added publish and unpublish to MRActions['Template']
• lib/adapters/REST/endpoints/template.ts — REST endpoint functions following the ComponentType publish/unpublish pattern
• lib/plain/entities/template.ts — method signatures added to TemplatePlainClientAPI
• lib/plain/plain-client.ts — wired via wrap()
• test/unit/adapters/REST/endpoints/template.test.ts — two unit tests verifying URL and X-Contentful-Version header for each method

This PR is based on feat/exo with the three open template PRs (#2966, #2967, #2969) merged in to avoid conflicts when they land.

Motivation and Context

Part of the incremental Template entity build-out for Experience Orchestration. Upstream contract: template.ts in assemblies.

Related tickets: DX-903, DX-904

PR Checklist

• I have read the CONTRIBUTING.md file
• All commits follow conventional commits
• Documentation is updated (if necessary)
• PR doesn't contain any sensitive information
• There are no breaking changes

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