perf: test nesting level only once

[de-sh]

Fixes parseablehq#1058.

Description

In this branch we perform the nesting level check at every level of nesting that we traverse with generic_flattening, which is unnecessary as a single test at the starting would be enough, which is what the change proposes to do.

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This PR has:

• been tested to ensure log ingestion and log query works.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added documentation for new or modified features or behaviors.