imap-parser-nesting-limit

[uwezkhan]

The IMAP parser currently tracks list_count_limit, but nested list depth is only constrained after lists are closed. This means deeply nested input can still build a large recursive imap_arg tree before the limit is enforced.

Several downstream imap_arg consumers traverse these structures recursively, so excessively deep nesting may lead to stack exhaustion.

This change adds proactive nesting-depth enforcement when opening lists by:

• tracking current nesting depth during parsing
• rejecting input once nesting exceeds the configured list_count_limit
• applying a hard safety ceiling of 100 levels to protect recursive consumers even if a higher limit is configured

The hard ceiling is intentionally conservative and aligns with similar nesting limits elsewhere in the codebase.

Tests are included for both:

• the default hard nesting ceiling
• custom list_count_limit enforcement

[cmouse]

fixed with 9a0f8c1