♻️ Simplify OpenQASM gate and entry-point naming

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Problem Statement

OpenQASM import normally names the generated entry function main. A custom gate may also be named main, so the importer renames the entry function to _mqt_entry and probes numbered suffixes against all custom gate names. This works, but spreads a source-name collision into the internal symbol policy and adds special-case code.

This follows up on the final review of #2338.

Proposed Solution

Define a simple internal naming contract that keeps the generated entry point distinct from source-level custom gate symbols. Preserve legal custom gate names, including main, across import and export while maintaining symbol-table uniqueness.

Remove the collision scan if the new invariant provides the same guarantee. Keep regression coverage for a custom gate named main and for collisions with reserved internal names.

Start by tracing the OpenQASM importer and exporter code that assigns the generated entry function name, then inspect the symbol-table uniqueness rules. Define the internal naming contract so custom gate names such as main remain legal while reserved internal names stay unique. Preserve regression coverage for both custom main and reserved-name collisions.