regexp/syntax: extract maxRepeat constant and document nested repetition limit

[kunwar-vikrant]

The repeat count limit of 1000 was hardcoded as a magic number in
multiple places. Extract it into a named constant maxRepeat for clarity.

Additionally, the documentation stated that repeat counts above 1000
are rejected, but did not mention that nested repetitions have their
counts multiplied. For example, (?:[a-z]{4}){0,251} is rejected
because the product 4*251=1004 exceeds 1000, even though neither
individual count does. Document this behavior.

Add test cases for the exact boundary: {4}{250} (product 1000, valid)
and {4}{251} (product 1004, invalid).

Fixes #78222

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