Fix simplifier visit budget exhaustion

[srkyn]

Regexp::Walker::Walk() can stop after its visit budget is exhausted, but the simplifier's two ShortVisit() implementations still log DFATAL while unwinding. This aborts fastbuild processes and produces repeated internal error logs in optimized builds.

Remove those stale DFATAL calls and keep the existing unwinding return values. Regexp::Simplify() already detects stopped_early() and returns the existing pattern-too-large error.

Added a regression that exhausts the walk budget and verifies the failure stays nonfatal.

Tested:

• the focused simplifier regression
• the surrounding RE2 test targets in fastbuild
• the same targets with Clang 18 and UBSan