fix: spill oversized tool errors

[PsiACE]

Apply spill limits to model-rendered tool results, including structured successes and failures, while preserving small original values and failure state. Adds regression coverage and updates the English and Chinese docs.

Verification:

• uv run ruff check .
• uv run mypy src
• uv run pytest -q (353 passed, 1 skipped)
• Astro docs build

[PsiACE]

str   -> text as-is -----+
dict  -> JSON text ------+
list  -> JSON text ------+--> size check
error -> as_dict -> JSON-+        |
                           +-----+-----+
                           |           |
                         small        large
                           |           |
                     keep original  spill full text
                           |           |
                           |     +---------------------------+
                           |     | [spilled: ...; handle: x] |
                           |     | [read: spill_read(...)]   |
                           |     | head ... omitted ... tail|
                           |     +---------------------------+
                           |           |
                           +-----+-----+
                                 |
                                tape
                                 |
                           model context

spill_read -> bounded text page (no re-spill, no JSON auto-parse)
error      -> remains error
preview    -> text only; may not be valid JSON