json_view: pass accept()'s validate-only request as a parameter

Address review of #5970:
- build() takes validate_only as an argument; the validate_only members of
  document_data and basic_json_document are removed, so neither grows
- the internal read path takes an on_error mode (discard, raise, validate)
  instead of allow_exceptions; accept() calls it with on_error::validate
- build_with() has no default for Store
- drop the unneeded static_casts in push_node()
- name the NulIsEnd argument so astyle keeps build<...> intact
- explain in the docs and code why comments/trailing commas keep the index

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N5NasF8Toq7uBWUMCrSjXV

docs/mkdocs/docs/api/basic_json_document/accept.md

@@ -13,12 +13,16 @@ function never throws an exception for invalid input, and the returned `#!cpp bo
 is returned.
 
 The result is the same as that of `BasicJsonType::accept()` by design; the difference is how the check is made. This
-function runs the document's builder, which checks the text as [`parse()`](parse.md) does but writes no index (for
-input without comments and trailing commas; with either option it builds the index and discards it), while
+function runs the document's builder, which checks the text as [`parse()`](parse.md) does, while
 `BasicJsonType::accept()` runs the SAX parser with a handler that ignores every event. Use this one if you go on to
 parse the same input with [`parse()`](parse.md) anyway, so that both use the same code; either is fine for a plain
 yes/no answer.
 
+When both `ignore_comments` and `ignore_trailing_commas` are `#!cpp false` (the default), the builder writes no index.
+With either option set, it builds the index as [`parse()`](parse.md) does and discards it: the result is the same, only
+the time differs. (Neither option needs the index; the variant that skips it exists only for the default options to
+keep the code size down.)
+
 ## Template parameters
 
 `InputType`