fix(copier): preserve executable file mode

[YusufKaya00]

Changes

• Preserve the owner executable bit when Copier returns added or modified artifacts.
• Apply the same mode detection to renamed files.
• Add regression coverage for executable and regular generated files.

Tested with:

• vitest run lib/modules/manager/copier/artifacts.spec.ts --coverage --coverage.include=lib/modules/manager/copier/artifacts.ts (17 tests; 100% statements, branches, functions, and lines for artifacts.ts)
• Prettier, Biome, and Oxlint on the changed files

Context

• This closes an existing Issue, Closes: Copier manager doesn't set isExecutable on added/modified files #42239
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

AI assistance disclosure

Did you use AI tools to create any part of this pull request?

• No — I did not use AI for this contribution.
• Yes — minimal assistance (e.g., IDE autocomplete, small code completions, grammar fixes).
• Yes — substantive assistance (AI-generated non-trivial portions of code, tests, or documentation).
• Yes — other (please describe):

OpenAI Codex (GPT-5) was used for codebase navigation, implementation, regression tests, local verification, and pull request wording.

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests, but ran on a real repository, or
• Both unit tests + ran on a real repository

The public repository: N/A

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