Stop CBOR indefinite-length strings from recursing per chunk

[nlohmann]

Closes one vector of #5104.

What

get_cbor_string() and get_cbor_binary() handled the indefinite-length forms (0x7F and 0x5F) by calling themselves once per chunk. Since a chunk may itself be an indefinite-length string, an input of repeated 0x7F bytes reached one native stack frame per input byte: 200,000 of them crash the process with SIGSEGV before a single byte is rejected.

This is the path that blocked #5293, and it is untouched by the container-level work in the rest of this stack.

How

Count the open levels instead of recursing through them. That is enough here because every chunk is appended to the same result — get_bytes() writes at result.size() — so there is no per-level state to keep. The temporary chunk string and its copy into the result go away with the recursion.

The definite-length cases move to get_cbor_string_chunk() / get_cbor_binary_chunk() unchanged, including their error messages, which still name 0x7F and 0x5F because those are handled one level up.

Verification

Compared against develop over the interesting byte sequences — empty, single-chunk, nested, over-closed and truncated forms, both strings and byte arrays, and an indefinite-length map key: identical values, error codes, messages and byte offsets. The 200,000-level input now reports parse_error.110 at byte 200001 instead of crashing.

Relationship to #5325

#5325 proposes rejecting nested indefinite-length chunks per RFC 8949 §3.2.3. This PR is orthogonal and behaviour-preserving: it removes the recursion without changing what is accepted, which keeps the cbor_binary.cbor fixture and its decoding assertions working. If #5325 is later accepted, it becomes a single condition on the level counter here rather than a change to the control flow.

API impact

No breaking changes. Same inputs accepted, same errors, same byte offsets.

Checklist

• 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.

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