🐛 Preserve attributes on reused QIR declarations

[simon1hofmann]

🤖 AI text below 🤖

Description

Preserves QIR runtime declaration attributes when an existing declaration is
reused.

getOrCreateFunctionDeclaration previously replaced the complete
passthrough array when adding the irreversible marker. The helper now merges
that marker into the existing array, preserves unrelated attributes, and avoids
duplicates across repeated calls.

This is one focused replacement for draft PR #2287 and addresses part of #2255.

Validation

• focused passthrough regression — passed
• full QIR IR suite — 117/117 passed
• uvx nox -s lint — passed
• git diff --check — passed

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
• I have added migration instructions to the upgrade guide (if needed).
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

If PR contains AI-assisted content:

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• Every agent-authored or agent-edited public text body begins with the visible disclosure 🤖 *AI text below* 🤖 (titles are exempt).
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• I confirm that I have personally reviewed and understood all AI-generated content, and accept full responsibility for it.

AI assistance disclosure: Codex extracted this focused change from the
authorized #2255 audit, validated it, and drafted this description.

[burgholzer]

Also LGTM 👍🏼

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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