[Player] fix default potion expressions

[Harmenszoon]

Summary

• Fix potion.<name> expressions so they use a player's class default potion when potion= is not explicitly set.
• Keep the generated default potion string alive while comparing through std::string_view.

Cause

player_t::create_expression() only consulted default_potion() when the default was empty, so common expressions such as potion.lights_potential_2 returned false for profiles relying on class defaults. The same code path also assigned a temporary std::string return value into a std::string_view.

Validation

• Rebuilt build-codex\simc.exe with MSVC/CMake.
• Minimal Beast Mastery Hunter repro, no explicit potion:
  • Before: actions=potion,if=potion.lights_potential_2 did not show lights_potential.
  • After: default and explicit potion=lights_potential_2 runs both show lights_potential and TotalEvents=59 with threads=1 target_error=0 deterministic=1 seed=12345.
• Negative control: actions=potion,if=potion.tempered_potion_3 remains false and does not show the potion buff.
• Suffix alias control: actions=potion,if=potion.lights_potential still reaches the suffix-stripping path and shows the potion buff.
• profiles/CI.simc iterations=10 cleanup_threads=1 output=NUL exits 0. Existing baseline warnings about Frostbane, Static Charge, Fury default talents, and runeforges remain unchanged.

Generated files

None.