feat(sandbox): truncate oversized bash and read_file tool outputs (#1677)

* feat(sandbox): truncate oversized bash and read_file tool outputs

Long tool outputs (large directory listings, multi-MB source files) can
overflow the model's context window. Two new configurable limits:

- bash_output_max_chars (default 20000): middle-truncates bash output,
  preserving both head and tail so stderr at the end is not lost
- read_file_output_max_chars (default 50000): head-truncates file output
  with a hint to use start_line/end_line for targeted reads

Both limits are enforced at the tool layer (sandbox/tools.py) rather
than middleware, so truncation is guaranteed regardless of call path.
Setting either limit to 0 disables truncation entirely.

Measured: read_file on a 250KB source file drops from 63,698 tokens to
19,927 tokens (69% reduction) with the default limit.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(tests): remove unused pytest import and fix import sort order

* style: apply ruff format to sandbox/tools.py

* refactor(sandbox): address Copilot review feedback on truncation feature

- strict hard cap: while-loop ensures result (including marker) ≤ max_chars
- max_chars=0 now returns "" instead of original output
- get_app_config() wrapped in try/except with fallback to defaults
- sandbox_config.py: add ge=0 validation on truncation limit fields
- config.example.yaml: bump config_version 4→5
- tests: add len(result) <= max_chars assertions, edge-case (max=0, small
  max, various sizes) tests; fix skipped-count test for strict hard cap

* refactor(sandbox): replace while-loop truncation with fixed marker budget

Use a pre-allocated constant (_MARKER_MAX_LEN) instead of a convergence
loop to ensure result <= max_chars. Simpler, safer, and skipped-char
count in the marker is now an exact predictable value.

* refactor(sandbox): compute marker budget dynamically instead of hardcoding

* fix(sandbox): make max_chars=0 disable truncation instead of returning empty string

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: JeffJiang <for-eleven@hotmail.com>

config.example.yaml

@@ -12,7 +12,7 @@
 # ============================================================================
 # Bump this number when the config schema changes.
 # Run `make config-upgrade` to merge new fields into your local config.yaml.
-config_version: 4
+config_version: 5
 
 # ============================================================================
 # Logging
@@ -366,6 +366,13 @@ sandbox:
   # trusted, single-user local workflows.
   allow_host_bash: false
 
+  # Tool output truncation limits (characters).
+  # bash uses middle-truncation (head + tail) since errors can appear anywhere in the output.
+  # read_file uses head-truncation since source code context is front-loaded.
+  # Set to 0 to disable truncation.
+  bash_output_max_chars: 20000
+  read_file_output_max_chars: 50000
+
 # Option 2: Container-based AIO Sandbox
 # Executes commands in isolated containers (Docker or Apple Container)
 # On macOS: Automatically prefers Apple Container if available, falls back to Docker