fix(langchain): recognize SDK system health flag

[r266-tech]

Description

viking_health prefers the SDK's get_status(), which returns is_healthy, but the adapter only recognized other health fields. Healthy and unhealthy SDK responses both became state: "unknown".

Recognize a boolean is_healthy after the existing healthy/ok and recognized status/state values, preserving their precedence. Include the flag in the existing safe summary; raw errors and component details remain excluded. Server health computation and SDK behavior are unchanged.

Human Involvement

• A human participated in the implementation or review loop
• This PR was generated entirely by AI agents without human participation in the loop

Related Issue

Fixes #5876.

Type of Change

• Bug fix (non-breaking change that fixes an issue)

Changes Made

Extend the existing public-tool safe-summary test to cover canonical booleans, conflicting legacy fields, and a non-boolean flag.

Testing

• Real loopback HTTP server → current SyncHTTPClient.get_status() → public viking_health: 10 controls / 20 HTTP requests. Unmodified main fails the two canonical boolean controls; this change passes all 10, including legacy precedence and safe-summary controls. This tests response consumption, not a deployed OpenViking backend or model service.
• python -m pytest --noconftest -o addopts= -q tests/unit/test_langchain*.py tests/integration/langchain_langgraph: 218 passed, 1 skipped, with 68 existing deprecation/future warnings. The skipped host-summary persistence test requires the optional langgraph.checkpoint.sqlite package, which is not installed in this isolated environment.
• ruff format --check and ruff check on both changed files: passed.
• mypy --config-file examples/langchain/pyproject.toml examples/langchain/src/langchain_openviking: passed (15 source files).
• git diff --check: passed.

Python 3.12.14 / macOS arm64. Hosted CI remains to be observed.

• I have added tests that prove my fix is effective
• New and existing component tests pass locally
• macOS
• Linux
• Windows

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• The existing safe-summary boundary and field precedence are preserved