fix(perf): hoist regexp.MustCompile outside loop in validateExpressionForDangerousProps

[JasonOA888]

Fixes #20026

Problem

Two regexp.MustCompile calls in validateExpressionForDangerousProps:

1. Line 212: regexp.MustCompile("[.\\[\\]]+") - called per function invocation
2. Line 216: regexp.MustCompile("^\\d+$") - called inside for loop, per iteration

Since validateSingleExpression is recursive and called for every expression in every compiled workflow, these allocations accumulate significantly.

Solution

Hoist both regex patterns to package-level var declarations alongside existing regex vars:

var (
    exprPartSplitRe   = regexp.MustCompile(`[.\[\]]+`)
    exprNumericPartRe = regexp.MustCompile(`^\d+$`)
)

Replace inline MustCompile calls with references to these pre-compiled patterns.

Impact

• Eliminates repeated regex compilation in hot validation path
• No functional changes, only performance optimization
• Follows existing pattern in the file (lines 66-79)

Reported in discussion #19993 (Sergo audit).

[pelikhan]

Please contribution guide.