Add early length validation in check_label

[metsw24-max]

Oversized labels can still reach expensive contextual validation through
lower-level label-processing entry points such as:

• alabel()
• ulabel()
• check_label()
• the incremental idna2008 codec

CONTEXTJ/CONTEXTO validation may scan the full label repeatedly so very large attacker-controlled labels can drive pathological O(n²) processing.

This patch adds an early length guard in check_label(), the shared validation path used by all label-processing APIs, rejecting oversized labels before expensive validation executes.

The implementation uses valid_string_length(..., trailing_dot=True) to preserve existing UTS #46 lenient handling behavior while still bounding worst-case processing time.

Tests are added to verify prompt rejection across:

• check_label()
• alabel()
• ulabel()
• the incremental idna2008 codec