fix(core): preserve initial messages when loading buffered contexts

[zoedsy]

Why are these changes needed?

Reloading a BufferedChatCompletionContext component with initial_messages turns its messages into dictionaries because _from_config() unpacks config.model_dump(). This breaks the LLMMessage return contract and prevents get_messages() from removing a leading function result after truncation.

Pass the validated message objects directly to the constructor, as the other context implementations do. Add JSON component round-trip regressions for ordinary messages and a truncated tool-call exchange.

Related issue number

No existing issue; reproduced through a JSON component round trip.

Validation

• Both new regression tests fail against the unchanged implementation.
• pytest tests/test_model_context.py -q: 11 passed.
• Ruff formatting and lint checks passed for both changed files.
• Mypy passed for both changed files.

Checks

• I've included any doc changes needed (no public API or documentation changes).
• I've added tests corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed (upstream CI pending).

This change and its regression tests were prepared with Codex.

[zoedsy]

@microsoft-github-policy-service agree