fix(sse): preserve ZWNJ and ZWJ in sanitized responses (#12359)

The response de-obfuscation stripped the whole U+200B..U+200D range, so Persian/Kurdish half-spaces (U+200C), Arabic/Indic shaping and emoji ZWJ sequences (U+200D) were deleted from every assistant response — text, reasoning and tool-call arguments, streaming and non-streaming, every provider: ارائه‌دهنده came back as ارائهدهنده.

The request side only ever inserts a U+200D between two ASCII word characters, so the new stripObfuscationZeroWidth() removes a joiner only there, or at a string edge next to one so a word split across streaming deltas is still cleaned; U+200B and U+FEFF keep their unconditional removal. All seven copies of the old regex now go through the helper.

Validated in a combined worktree with all 25 PRs of this batch boarded together: typecheck:core clean, 443/443 node-runner tests plus 14/14 vitest across every test file the batch touches, and check-changelog-integrity, check:cycles (418 files), check:provider-consistency (272 REGISTRY entries, 355 canonical providers), check:docs-counts, check:docs-sync (42 locales) and check-file-size all green.

Thanks @pacocartones.