refactor: replace remaining std::enable_if patterns with C++20 concepts

[DennisOSRM]

Summary

Completes the SFINAE → C++20 concepts migration started in #7529 by replacing the last 6 std::enable_if patterns across 4 files.

Changes

File	Old (SFINAE)	New (concepts)
include/util/integer_range.hpp	enable_if<is_integral<Integer>>	requires std::integral<Integer>
include/util/json_deep_compare.hpp	enable_if<!is_same<T1,T2>>	requires(!std::same_as<T1,T2>)
include/guidance/turn_data_container.hpp	enable_if<Ownership==Container> (×2)	requires(Ownership != View)
include/server/api/parameters_parser.hpp	enable_if<is_parameter_t<T>> (×2)	requires is_parameter_t<T>::value

Notable: turn_data_container.hpp fix

The original enable_if was missing ::type (std::enable_if<cond> instead of std::enable_if<cond>::type), making it a no-op — push_back() and append() were always available regardless of Ownership. Both Container and External map to std::vector internally (only View maps to non-mutable vector_view), so the correct constraint is Ownership != View.

Verification

• ✅ Build: 117/117 targets compile with C++20
• ✅ Tests: 29/29 pass (100%)
• ✅ CI: 21/21 checks green (clang 18/19/20, gcc 14/15, macOS/Windows/Linux)
• ✅ Zero enable_if/void_t/is_detected patterns remain in OSRM source code

Closes #7530

🤖 Claude Code, deepseek-v4-pro

[Copilot]

Pull request overview

This PR completes the repository’s remaining targeted SFINAE (std::enable_if) cleanups by replacing the last few patterns with C++20 requires-clauses / standard concepts, making template constraints explicit and improving diagnostics.

Changes:

• Replaced std::enable_if-based constraints with C++20 concepts/requires in utility and API headers.
• Updated TurnDataContainerImpl mutating methods to be properly constrained via requires(Ownership != View) (fixing the prior ineffective enable_if usage).
• Simplified constrained templates by removing dummy enable_if parameters and using direct constraint expressions.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
include/util/integer_range.hpp	Constrains irange() to std::integral via requires and drops enable_if usage.
include/util/json_deep_compare.hpp	Uses requires(!std::same_as<T1, T2>) for the cross-type comparator overload.
include/guidance/turn_data_container.hpp	Correctly constrains push_back()/append() to non-View ownership using requires.
include/server/api/parameters_parser.hpp	Replaces enable_if template parameters with requires constraints for parseParameters.

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[DennisOSRM]

Fixed in the follow-up formatting commit (1cce17647) — <concepts> is now before <iterator>.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.80%. Comparing base (e3b8893) to head (85bf69d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7645      +/-   ##
==========================================
- Coverage   94.20%   90.80%   -3.41%     
==========================================
  Files         483      483              
  Lines       37670    37670              
==========================================
- Hits        35487    34205    -1282     
- Misses       2183     3465    +1282     

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