Reject nested indefinite-length CBOR string chunks

[JosephDemarest]

Summary

• enforce RFC 8949 Section 3.2.3 by allowing only definite-length chunks inside indefinite-length text and byte strings
• add regression coverage for both CBOR major types and update the existing nested-byte-string fixture expectation
• regenerate the amalgamated header and document the chunk requirement

Closes #5317.

Validation

• ctest --test-dir build17 -j 8 -LE "not_reproducible|git_required" --output-on-failure — 77/77 passed (Clang 20, C++17)
• focused test-cbor_cpp17 and test-regression2_cpp17 targets — passed
• compiled and ran the malformed-input check against single_include/nlohmann/json.hpp — both inputs raised parse_error.113 at byte 2
• Artistic Style 3.4.13 and git diff --check — clean

Checklist

• The changes are described in detail, both the what and why.
• Existing issue CBOR: nested indefinite-length strings/byte strings are accepted (RFC 8949 3.2.3 requires definite-length chunks) #5317 is referenced.
• Regression tests cover every new branch.
• The CBOR documentation is updated.
• The source code is amalgamated.

[nlohmann]

This can yield confusing error messages like

expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0x7F

Maybe we should rather have something like

concat("indefinite-length string is not allowed inside indefinite-length string; last byte: 0x", last_token)

or just drop the or indefinite string type (0x7F).