style(runtime-fallback): add explicit optional chain on .replace per review

Addresses cubic-dev-ai P1 finding on #4113 (#4113 review).

The original chain `extractErrorName(error)?.toLowerCase().replace(...)`
is semantically safe — JavaScript optional chaining short-circuits the
ENTIRE access chain when the head returns null/undefined, so when
`extractErrorName` returns undefined the whole expression evaluates to
undefined without ever reaching `.replace()`. Verified empirically via
`const x = undefined; x?.toLowerCase().replace(/_/g, "")` returns
undefined with no crash.

Applying the suggested defensive `?.` before `.replace` anyway, since
it is semantically a no-op and explicit chaining at each hop is easier
for static analyzers to reason about.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/hooks/runtime-fallback/error-classifier.ts

@@ -109,7 +109,7 @@ export function classifyErrorType(error: unknown): string | undefined {
   // Normalize by stripping underscores and dashes so snake_case / kebab-case
   // provider error names (e.g. "insufficient_quota", "RESOURCE_EXHAUSTED")
   // match the existing alphanumeric .includes() checks below.
-  const errorName = extractErrorName(error)?.toLowerCase().replace(/[_-]/g, "")
+  const errorName = extractErrorName(error)?.toLowerCase()?.replace(/[_-]/g, "")
 
   if (
     errorName?.includes("ailoadapikeyerror") ||