More type annotations, to_float() default fix

[zsimic]

Found while bringing pickley to zero type-checker findings:

• Better type annotations: to_int(), to_float() (overloads on default), json_sanitized() (dict in -> dict out, sequence in -> list out), ClickRunner.context_wrapper (any context manager class) and ClickRunner.context
• Bug fix: to_float() (and so Configuration.get_float()) returned default for any non-integer string when a default was given: to_float("1.5", default=0) yielded 0.0
• Version.mm and Version.main are now empty strings (instead of None) when not applicable, so they're typed str (callers already gate with if v.mm:, and Version("") == Version(None))

ty, pyrefly, pyright and mypy remain at zero, coverage at 100%.

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