feat: add URL parser foundation

[zuudevs]

Summary

Starts the first v1 implementation slice by adding the foundational public error/result types and a tested HTTP URL parser.

Added

• include/cpp_request/error.hpp
  • frozen ErrorCode taxonomy from the v1 error-model docs
  • lightweight Error with optional native diagnostic code
  • allocation-free error_message()
• include/cpp_request/result.hpp
  • C++17 Result<T> backed by std::variant<T, Error>
  • [[nodiscard]], has_value(), explicit operator bool(), value(), and error()
• include/cpp_request/url.hpp
  • parsed URL value type
  • scheme/host/path/query/target views
  • effective port and explicit-port metadata
  • IPv6-literal metadata
• src/url.cpp
  • http:// parsing
  • case-insensitive HTTP scheme recognition with normalization
  • default port 80
  • explicit port parsing and validation
  • bracketed IPv6 literal support
  • default / path
  • query handling
  • fragment exclusion from request-target
  • rejection of unsupported schemes and v1 user-info
• tests/url_test.cpp
  • happy-path and invalid-input coverage

Build changes

• update project baseline from C++11 to the frozen C++17 requirement
• compile URL parser into the internal target
• wire GoogleTest into tests/

Scope

This PR intentionally does not implement DNS, sockets, HTTP serialization, response parsing, or Client yet. It establishes the value/error and URL foundation those layers will build on.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Restore a fallback before requiring GTest

On any clean build environment without a preinstalled GTest CMake config, configuration now stops at this line; this includes the checked .github/workflows/ci.yml path, which runs plain cmake -B build without activating the repository's vcpkg toolchain or installing GTest. The repository already has cmake/packages/google-test.cmake to fetch GTest when it is unavailable, so use that bootstrap or otherwise make tests optional instead of making a test-only dependency mandatory for every library configuration.

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[chatgpt-codex-connector]

Enable CTest before registering the URL tests

When GTest is available, this registration is still ineffective because no active CMake path calls enable_testing() or includes CTest; the only existing call is in the unused cmake/packages/google-test.cmake. As documented by cmake --help-command enable_testing, testing must be enabled from the source root, and configuring this tree with a GTest config followed by ctest -N reports Total Tests: 0, so these URL tests will never run through CTest.

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[chatgpt-codex-connector]

Validate bracketed hosts before marking them as IPv6

For any nonempty bracket contents, the parser reports success and sets host_is_ipv6_literal_ without checking IPv6 syntax. For example, Url::parse("http://[example.com]/") and Url::parse("http://[:::]/") both succeed and are labeled IPv6 literals even though those are not valid IPv6 addresses, violating the parser's structured invalid-URL behavior; validate the bracketed address before accepting it and setting this metadata.

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