refactor: Remove args-wrappers in favor of naked value_or() #36295

Decreases complexity at the foundation at the only cost of a change to ArgsManager::GetPathArg_() and slightly more verbose code in one leaf; src/qt/optionsmodel.cpp.

Also represents slight decrease in complexity of pending PR #31260 which has an extra GetPtr() function for disambiguating between overloaded functions.

Original thread: #31260 (review)

(Rebased it onto the current base commit of #31260 and rebased #31260 on top of that, removed GetPtr(), and confirmed it still compiles: https://github.com/hodlinator/bitcoin/tree/pr/31260_no_GetPtr).

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36295.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #17783 (common: Disallow calling IsArgSet() on ALLOW_LIST options by ryanofsky)
• #17581 (refactor: Remove settings merge reverse precedence code by ryanofsky)
• #17580 (refactor: Add ALLOW_LIST flags and enforce usage in CheckArgFlags by ryanofsky)
• #17493 (util: Forbid ambiguous multiple assignments in config file by ryanofsky)
• #16545 (refactor: Implement missing error checking for ArgsManager flags by ryanofsky)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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lightly tested code review ACK 60e95b412c999aa9efe561c99994f8e53d5ab4f6

Pure refactor, the removed helpers were thin wrappers that just delegated to the existing std::optional-returning overloads via value_or() - inlining them simplifies the interface.

Concept ACK