fix(core): count dropped function call messages in head-and-tail skipped placeholder

[madanmishra1223]

Why are these changes needed?

HeadAndTailChatCompletionContext.get_messages() deliberately drops two kinds of message so the returned view never contains a dangling half of a function call pair:

• a trailing AssistantMessage whose content is a list of FunctionCalls, removed from the head
• a leading FunctionExecutionResultMessage, removed from the tail

The "Skipped N messages." placeholder that replaces the omitted middle was computed before those drops were taken into account:

num_skipped = len(self._messages) - self._head_size - self._tail_size

That expression only counts the messages between the head and tail windows. When either guard fires, one or two additional messages are dropped but never counted, so the placeholder understates how much history was removed — by one if a single guard fires, by two if both do.

The placeholder is a UserMessage that goes to the model, so this is wrong information in the prompt: the model is told less context was elided than actually was.

Repro

6 messages, head_size=2, tail_size=2, where messages[1] is a tool-call AssistantMessage (trimmed from the head) and messages[4] is a tool-result message (trimmed from the tail):

returned messages:
    UserMessage 'm0 user'
    UserMessage 'Skipped 2 messages.'
    UserMessage 'm5 user'

total messages   : 6
kept (non-placeholder): 2
actually omitted : 4
placeholder says : 'Skipped 2 messages.'

Two messages are kept out of six, so four were omitted — the placeholder says two.

What changed

Derive the count from the head and tail lists that are actually returned:

num_skipped = len(self._messages) - len(head_messages) - len(tail_messages)

The early return for the "not enough messages to fill head and tail" case still uses the configured head_size/tail_size, so a context that needs no truncation keeps returning all messages unchanged. Only the placeholder count changes, and only when a truncating call also dropped a function call message.

After the fix the same repro reports Skipped 4 messages.

Checks

• Added test_head_and_tail_model_context_placeholder_counts_dropped_function_call_messages, which covers the both-guards-fire path that existing tests did not reach. It fails on main and passes with this change.
• pytest packages/autogen-core/tests — 227 passed.
• pytest packages/autogen-agentchat/tests/test_agent.py packages/autogen-agentchat/tests/test_declarative_components.py (the dependents that construct this context) — 14 passed.
• ruff format --check, ruff check, and mypy clean on both touched files.

Related issue number

None — found by inspection.

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

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