ext/strings: add runtime cost tracking for string extension functions

[MindflareX]

Summary

The string extension library (ext/strings.go) does not register any OverloadCostTracker entries in its ProgramOptions() method, causing all string extension function calls to fall through to the default case in costCall() (interpreter/runtimecost.go:337) which charges a cost of 1 regardless of input size or output size.

This is inconsistent with ext/lists.go, ext/sets.go, and ext/regex.go which all register proper cost trackers.

Impact

The missing cost tracking allows resource exhaustion when CostLimit() is used as a defense mechanism:

• indexOf()/lastIndexOf(): Uses an O(N×M) naive nested-loop search implementation (ext/strings.go:608-619). A 500K string searched against a 200K almost-matching pattern burns 24 seconds of CPU at CostLimit(100) with no error.

• join(): Concatenates N strings into a single output string. Joining 10,000 strings of 10,000 chars each allocates 587 MB at CostLimit(100) with no error.

• replace(): Output can be significantly larger than input (e.g., replacing each character with a long string). replace("a", "<1000 chars>") on 100K input produces 95 MB with no cost error.

• lowerAscii()/upperAscii()/reverse()/split()/trim(): All perform O(N) work on the input string but are tracked as cost=1.

Fix

This change adds OverloadCostTracker registrations for all string extension functions. The cost formulas follow the patterns already used by built-in string operations in interpreter/runtimecost.go:

Function	Cost Formula
indexOf/lastIndexOf	str_size * substr_size * StringTraversalCostFactor
join/replace	result_size * StringTraversalCostFactor
lowerAscii/upperAscii/reverse/trim	input_size * StringTraversalCostFactor
split	result_size * StringTraversalCostFactor + ListCreateBaseCost

Test

Added TestStringsCostTracking which verifies:

• Large indexOf/lastIndexOf/join operations correctly exceed cost limits
• Small operations remain within cost limits

All existing tests pass.

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[TristonianJones]

There's already an open PR for this which hasn't yet responded to feedback