fix(python-sdk): preserve Unicode separators in harness JSONL - #1148

Summary

Preserve valid JSONL events containing literal U+0085, U+2028, or U+2029 in CLI output. str.splitlines() treats those characters as record boundaries even inside JSON strings, so the shared parser silently discards the event. An OpenCode text event can consequently disappear while the provider reports success with no result.

Split records on LF as specified by JSON Lines, retaining CRLF support and the existing handling of blank or malformed lines. This fixes the shared parsing path used by the OpenCode, Codex, and Pi providers.

Type of change

• Bug fix

Test plan

• Regression matrix: all three Unicode separators with LF and CRLF framing failed before the fix.
• Shared CLI helper and affected provider suites: 106 passed.
• Local executable fixture through OpenCodeProvider.execute(): before the fix, result=null and 2 events; after the fix, exact text preserved and all 3 events. No model/API calls.
• ./scripts/test-all.sh: Control Plane and Go SDK passed in the cached Linux Go environment; Python SDK 2948 passed / 4 skipped / 38 deselected; TypeScript SDK 1383 passed / 1 skipped; Web UI 837 passed.
• cd sdk/python && ruff check .
• cd sdk/python && mypy --config-file mypy.ini agentfield/: 86 source files passed.
• make fmt tidy: completed; baseline-only formatting changes were reverted to keep this PR focused. No module files changed. Full tests then ran on the final tree.
• Registered additional checks: memory-event suites with websockets 12 and 15, 28 passed each; upstream Python memory benchmark passed.

The repository's default exclusions for live/functional tests were retained. Web UI lint is opt-in in test-all.sh and was not enabled for this Python-only change.

Test coverage

• Added regression coverage for every changed behavior, including both supported line endings.
• Repository coverage CI result pending.

Checklist

• Read docs/CONTRIBUTING.md and docs/DEVELOPMENT.md.
• Focused conventional commit with sign-off.