Raise line coverage to 99.9 %: fix the measurement, remove unreachable code, add tests (#5997)

* CI: measure line coverage without lcov's unexecuted-block adjustment

Drop geninfo_unexecuted_blocks=1 from the lcov capture of ci_test_coverage:
it reset every line of an inlined or templated function to 0 as soon as one
copy had an unexecuted block, which understated the line coverage by about
three points.

Add -fvar-tracking-assignments to both coverage builds. At -O0, GCC inlines
always_inline functions before it places the coverage counters, so the call
sites of such helpers (put_literal() and others) got no line record and
showed as 0 in an -O3 translation unit that never reaches them. With the
flag, the call sites carry counts in every translation unit. The generated
code is identical.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Remove unreachable code found by the coverage analysis

Remove code that no input can reach, and mark the lines that cannot be
covered with LCOV_EXCL_LINE:

- BJData writer: the dry_run parameter of the ND-array element writers was
  always true; replace them by range checks.
- BON8 writer: call throw_integer_out_of_range() directly instead of
  write_bon8_value() for values that always throw.
- binary_writer: integer_bits() is only called for integers of up to 64 bits.
- serializer, view serializer, lexer: replace guards that the callers make
  unreachable by assertions or by the simpler code (surrogate pairs under
  ensure_ascii, next_unget, '-' at the end of the buffer).
- json_view: drop unreachable branches in scalar(), set(), check_utf8_scan(),
  append_input_impl() and unescape().
- Mark exception-cleanup braces, 2 GiB limits, big-endian and 32-bit code, and
  fall-through case labels with LCOV_EXCL_LINE.

No behavior changes for any input that reaches these functions.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Tests: cover the remaining lines of the library headers

Add tests for the lines of include/nlohmann that were not covered: the
binary readers and writers, the lexer and locale paths, the json_view
modules, the custom number, key and array types, json_pointer, json_patch,
diff and the iterators.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Fix test failures in the coverage tests

- unit-custom-object-key-type: remove the unused make_strict_key(strict_key)
  overload, the unused range constructor and default constructor, and guard
  data() with JSON_DIAGNOSTICS (unused-function/-member-function errors with
  GCC and clang -Weverything).
- unit-json_view_compare: move the two new sections to the new
  unit-json_view_compare_values.cpp, as the object file got too large for the
  MinGW assembler again, and use a std::map with std::greater instead of
  std::unordered_map, which does not accept the incomplete basic_json
  (clang 3.9, icpc).
- unit-json_view_dump: rename the variables that hid outer ones (MSVC C4456).
- unit-stream-bulk-scan: restore the real size of the oversized get area
  before the adapter is destroyed (JSON_PRECISE_STREAM_POSITION reads the
  character at the read position), and skip the test where the streambuf
  cannot hold a get area larger than an int (MSVC).
- unit-regression1: with JSON_NO_THREAD_LOCAL, copies are built member by
  member and keep correct timestamps, so check the corresponding outcome.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N5NasAgbDGG28MqXjQvLqp
Signed-off-by: Claude <noreply@anthropic.com>

* Return std::size_t from the hash helper in the ordered_json test

MSVC (Win32) rejected the implicit conversion of the unsigned long long
product to the 32-bit std::size_t (C4244 as an error). Also note when
JSON_NO_THREAD_LOCAL is set in the regression test.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N5NasAgbDGG28MqXjQvLqp
Signed-off-by: Claude <noreply@anthropic.com>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Signed-off-by: Claude <noreply@anthropic.com>
Co-authored-by: Claude <noreply@anthropic.com>

include/nlohmann/detail/json_pointer.hpp

@@ -304,7 +304,7 @@ class json_pointer
         result.reference_tokens.insert(result.reference_tokens.end(), lhs.reference_tokens.begin(), lhs.reference_tokens.end());
         result.reference_tokens.insert(result.reference_tokens.end(), rhs.reference_tokens.begin(), rhs.reference_tokens.end());
         return result;
-    }
+    } // LCOV_EXCL_LINE (exception cleanup only)
 
     /// @brief create a new JSON pointer by appending the unescaped token at the end of the JSON pointer
     /// @sa https://json.nlohmann.me/api/json_pointer/operator_slash/
@@ -315,7 +315,7 @@ class json_pointer
         result.reference_tokens.insert(result.reference_tokens.end(), lhs.reference_tokens.begin(), lhs.reference_tokens.end());
         result.reference_tokens.push_back(std::move(token));
         return result;
-    }
+    } // LCOV_EXCL_LINE (exception cleanup only)
 
     /// @brief create a new JSON pointer by appending the array-index-token at the end of the JSON pointer
     /// @sa https://json.nlohmann.me/api/json_pointer/operator_slash/
@@ -2180,7 +2180,7 @@ class json_pointer
                     {
                         // create an entry in the array
                         current = &current->operator[](array_index<BasicJsonType>(token));
-                        break;
+                        break; // LCOV_EXCL_LINE (needs an array index with 10 or more digits: more than 1e9 elements)
                     }
 
                     /*
@@ -2217,7 +2217,7 @@ class json_pointer
         json_pointer<string_t> result;
         result.reference_tokens = reference_tokens;
         return result;
-    }
+    } // LCOV_EXCL_LINE (exception cleanup only)
 
     json_pointer<string_t> convert()&&
     {