Add compile-time error message when user tries to read XML into generics; resolves #614

[liuzicheng1987]

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[gemini-code-assist]

Summary of Changes

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This pull request refines the handling of rfl::Generic types within the XML serialization and deserialization framework. It prevents unsupported operations at compile time, explicitly enables supported ones, and improves data type parsing robustness. The changes ensure a more predictable and reliable experience when working with XML and generic data structures.

Highlights

• Compile-time Error for XML Generic Reading: Introduced a compile-time error using static_assert to prevent users from attempting to read XML into rfl::Generic types, resolving a potential runtime issue.
• XML Generic Writing Support: Explicitly enabled and implemented the functionality for writing rfl::Generic types to XML, ensuring that this use case is supported.
• Improved Boolean Parsing in XML Reader: Enhanced the XML reader to correctly parse boolean values, now accepting '0' and '1' in addition to 'true' and 'false', and providing a clearer error message for invalid boolean strings.
• Documentation Update: Updated the docs/generic.md file to reflect the current support status of rfl::Generic with XML, clarifying that reading is not supported but writing is.
• New Test Case for XML Generic Writing: Added a dedicated test file to validate the correct serialization of rfl::Generic objects into XML format.

Changelog

• docs/generic.md
  • Updated documentation to clarify that XML generics are supported for writing but not for reading.
• include/rfl/xml/Parser.hpp
  • Added Generic.hpp and always_false.hpp includes.
  • Introduced a template specialization for rfl::parsing::Parser for rfl::Generic types with xml::Reader and xml::Writer.
  • Implemented a static_assert in the read method of the generic XML parser to generate a compile-time error when attempting to read XML into rfl::Generic.
  • Provided a write method for rfl::Generic to serialize generic objects to XML.
  • Added a to_schema method for rfl::Generic to generate schema for generic objects.
• include/rfl/xml/Reader.hpp
  • Included Generic.hpp.
  • Refactored boolean parsing logic to accept '1' and '0' as valid boolean representations in addition to 'true' and 'false'.
  • Added an error return for invalid boolean string values during parsing.
• tests/xml/test_generic.cpp
  • Added a new test file.
  • Implemented a test case to verify the correct XML serialization of an rfl::Generic::Object.

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Code Review

This pull request successfully implements a compile-time error for attempts to read XML into rfl::Generic, which is not supported. This is done using a static_assert in a specialized parser template. The PR also includes a useful improvement to boolean parsing from XML, allowing '0' and '1' as valid values, and modernizes some namespace declarations. A test is added to confirm that writing rfl::Generic to XML works as expected. The changes are well-executed. I have one minor suggestion to remove an unnecessary header include for better code hygiene.