check all BSON reads and add an EOF check for booleans

[xevrion]

Fixes #5309.

The BSON reader discarded the return values of three get_number() calls (the document size in parse_bson_internal and parse_bson_array, and the subtype in get_bson_binary) and read the boolean value with a bare get() that has no EOF check. As the issue notes, no truncated input escapes as an accepted document, but the failures surface downstream instead of at the failed read: a boolean truncated at EOF emits boolean(true) to the SAX handler before the element-list check reports the error, and a binary truncated at the subtype byte calls set_subtype() and get_binary() after its own parse error has already fired.

The two document-size reads now short-circuit with a plain if (!...) rather than JSON_HEDLEY_UNLIKELY, because that macro takes a single argument and the comma in get_number<std::int32_t, true> would be parsed as a macro argument separator; this matches the existing unwrapped call sites for the string, binary, and int32 element types. The subtype read keeps the JSON_HEDLEY_UNLIKELY guard, and the boolean is routed through get_number<std::uint8_t> so it gets the same EOF handling as every other element type.

Error messages for these truncated inputs now name the failed read (for example "BSON number" at byte 8 instead of "BSON element list" at byte 9). The existing "Incomplete BSON Input 2" assertion is unaffected because that input fails earlier, in get_bson_cstring. Valid input parses identically.

• The changes are described in detail, both the what and why.
• If applicable, an existing issue is referenced.
• The Code coverage remained at 100%. A test case for every new line of code.
• If applicable, the documentation is updated.
• The source code is amalgamated by running make amalgamate.

Two new sections, "Incomplete BSON Input 5" and "Incomplete BSON Input 6", cover the boolean and subtype paths; the document-size branches are exercised by the existing "Incomplete BSON Input 4" and the empty-input cases. No documentation change is needed.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks!