fix(core): trim oldest token-limited context messages

[pragnyanramtha]

Summary

Fixes #6789.

• trim oldest messages first in TokenLimitedChatCompletionContext instead of removing from the middle
• remove leading orphan FunctionExecutionResultMessage entries exposed by trimming
• add regression coverage that preserves assistant tool-call/function-result adjacency

Testing

cd python
uv run --with ollama pytest packages/autogen-core/tests/test_model_context.py -q
uv run ruff check packages/autogen-core/src/autogen_core/model_context/_token_limited_chat_completion_context.py packages/autogen-core/tests/test_model_context.py
uv run pyright packages/autogen-core/src/autogen_core/model_context/_token_limited_chat_completion_context.py packages/autogen-core/tests/test_model_context.py
git diff --check

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