fix(langchain): preserve system messages during recall injection

[ktz03]

Description

When recall context is injected, an existing typed SystemMessage is rebuilt with only flattened text. This drops its name, ID, additional fields and structured content blocks; a downstream filter_messages(include_names=[...]) can then omit the original named policy.

Preserve the existing message with model_copy, append one text block to list content, and keep the existing string formatting and caller input 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 #6097

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• Retain existing typed system-message fields when adding recalled context.
• Preserve original content blocks and their fields without mutating the caller's message.
• Add eight public runnable regressions for sync/async, string/block content and disabled/enabled injection.

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

The eight regressions are 4 failed / 4 passed on exact main and 8 passed with the fix. The scoped LangChain suites are Linux 231 passed / 1 SQLite skip; Windows 229 passed / 1 skip / 2 existing resource-path failures. Exact main with the same regressions is Windows 225 passed / 1 skip / 6 failures, including the four new regressions. Both Windows runs have the same two resource-path failures.

Ruff check/format (44 files), mypy (15 sources), wheel/sdist build, Twine, offline target install, the package's installed smoke and four installed public controls passed. Independent public name-filter controls are 3 normal / 3 divergent before the fix and all 6 normal after it. These overlapping selections are not added together.

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Screenshots (if applicable)

Not applicable.

Additional Notes

Validation used cached dependencies and the deterministic in-memory backend. Existing LangChain deprecation warnings remain. No deployed backend, live model, authentication, full-server, minimum-version matrix, provider cache-hit or performance claim is made.

The original integration/shared recording work by ehz0ah and host-history work by krsish remain intact. System messages remain excluded from persisted memory. This does not change the package layout proposed in #6017.