Make std::formatter usable with exceptions disabled

[cleishm]

Problem

The std::formatter specializations' parse() reported an unusable format spec (a precision with no SI prefix, formatted with an empty spec) with a bare throw format_error(...). Under -fno-exceptions — the default on embedded targets such as ESP-IDF with CONFIG_COMPILER_CXX_EXCEPTIONS unset — a throw in any instantiated function is a hard compile error, so even a perfectly valid std::format("{}", celsius(20)) failed to compile.

Fix

• The error branch is discarded via if constexpr when the precision has an SI prefix, so it is never instantiated for formatters that cannot need it.
• When it is needed, the error is reported through a new _format_error() helper: it throws std::format_error when exceptions are available, and calls std::abort() when they are not. Because the helper is not constexpr, consteval format-string checking still rejects a bad spec at compile time in both modes; the abort is only reachable through a runtime format string (std::vformat) with exceptions disabled, where there is no way to report the error.

Tests

• New tests/no_exceptions_test.cpp: a framework-free target compiled with -fno-exceptions exercising both formatters with empty and explicit specs (Catch2 requires exceptions, so it cannot live in thermo_tests). Skipped on MSVC, which spells the option differently and declares __cpp_exceptions regardless.
• Verified the prefix-less empty-spec case still fails to compile with and without exceptions.
• Full suite passes: 31/31.

Bumps version to 2.1.1 (CMakeLists, idf_component.yml, docs/Doxyfile, vcpkg-port/vcpkg.json).