Fix bundle init error for symlinked template directories

[TangoEnSkai]

Changes

Detect symlinks to directories while walking bundle templates and return an actionable error before writing output. Add unit and acceptance coverage and a changelog fragment.

Why

bundle init treated a symlinked directory as a file, failed with an internal read error, and left a partial output directory behind. This change reports that symlinked directories are unsupported and avoids partial output. Fixes #6391.

Tests

• go test ./libs/template
• go test ./acceptance -run TestAccept/bundle/templates-machinery/symlinked-directory -v
• ./task fmt
• ./task lint
• ./task checks
• ./task test passed all 9,475 unit tests; the full acceptance run was blocked only by the unrelated existing bundle/templates-machinery/supported-url SSH timeout in this local network.

This PR was written with OpenAI Codex.

[github-actions]

An authorized user can trigger integration tests manually by following the instructions below:

Trigger:
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Inputs:

• PR number: 6405
• Commit SHA: 0a781ec754d1bbdb7d078e3fed4d694d459e5cb0

Checks will be approved automatically on success.

[github-actions]

Approval status: pending

/acceptance/bundle/ - needs approval

7 files changed
Suggested: @denik
Also eligible: @janniklasrose, @andrewnester, @pietern, @lennartkats-db, @shreyas-goenka, @anton-107

/libs/template/ - needs approval

Files: libs/template/renderer.go, libs/template/renderer_test.go
Suggested: @denik
Also eligible: @janniklasrose, @andrewnester, @pietern, @lennartkats-db, @shreyas-goenka, @anton-107

General files (require maintainer)

Files: .nextchanges/bundles/template-directory-symlink.md
Based on git history:

• @denik -- recent work in .nextchanges/bundles/, libs/template/

_{Any maintainer (@andrewnester, @anton-107, @denik, @pietern, @shreyas-goenka, @simonfaltum, @renaudhartert-db, @janniklasrose, @lennartkats-db, @rugpanov, @rclarey) can approve all areas.
See OWNERS for ownership rules.}