fix(translator): drop unsigned thinking blocks instead of fabricating a Claude signature (#12386)

A thinking content part arriving with no signature — typical after a cross-provider hop where reasoning_content was converted into a thinking block — was stamped with DEFAULT_THINKING_CLAUDE_SIGNATURE. prepareClaudeRequest treats any non-empty signature on the latest assistant turn as genuine and preserves it verbatim, so the fabricated one reached Anthropic and the replay failed with "Invalid signature". A missing signature is now treated the same as an empty one, aligned with the stricter check claudeHelper.ts already used: the block is dropped rather than fabricated. Real signatures are still preserved verbatim and redacted_thinking is unchanged.

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.