ext/strings: add cost estimators for string extension functions

[mohammadmseet-hue]

Summary

The ext.Strings library registers 13+ string manipulation functions without any compile-time cost estimators or runtime cost trackers. All string extension functions fall through to a default O(1) cost in both the static cost checker (checker/cost.go) and runtime cost tracker (interpreter/runtimecost.go), regardless of actual input size or computational complexity.

Every other CEL-GO extension library registers cost estimators: ext/lists.go has 28, ext/regex.go has 15, ext/sets.go has 12. ext/strings.go has zero.

This means cel.CostLimit() is ineffective for expressions using string extension functions. For example, chaining split("") and join("XXX") calls can amplify a small input into significant memory allocation while the cost tracker reports a trivially low cost.

Changes

• Added checker.OverloadCostEstimate entries in CompileOptions() for all string extension overloads
• Added interpreter.OverloadCostTracker entries in ProgramOptions() for all string extension overloads
• Cost estimation follows the same patterns used by ext/regex.go and ext/lists.go:
  • O(n) traversal cost for simple operations (charAt, lowerAscii, upperAscii, trim, reverse, substring)
  • O(n*m) search cost for indexOf/lastIndexOf
  • Proportional cost with output size tracking for replace, split, and join
• strings.quote and string.format are excluded as they are already tracked by the default runtime cost calculator via overloads.ExtQuoteString and overloads.ExtFormatString

Affected overloads

Overload ID	Previous Cost	New Cost
string_char_at_int	O(1)	O(n)
string_index_of_string	O(1)	O(n*m)
string_index_of_string_int	O(1)	O(n*m)
string_last_index_of_string	O(1)	O(n*m)
string_last_index_of_string_int	O(1)	O(n*m)
string_lower_ascii	O(1)	O(n)
string_upper_ascii	O(1)	O(n)
string_replace_string_string	O(1)	O(n) + result size
string_replace_string_string_int	O(1)	O(n) + result size
string_split_string	O(1)	O(n) + list allocation
string_split_string_int	O(1)	O(n) + list allocation
string_substring_int	O(1)	O(n)
string_substring_int_int	O(1)	O(n)
string_trim	O(1)	O(n)
string_reverse	O(1)	O(n)
list_join	O(1)	O(n) + result size
list_join_string	O(1)	O(n) + result size

Test plan

• All existing tests pass (go test ./ext/... and go test ./...)
• No changes to test expectations required — the strings.quote tests are unaffected since quote/format are handled by the default runtime cost calculator