fix: remove orphaned FunctionExecutionResultMessages after truncation

[abhi-0203]

Summary

TokenLimitedChatCompletionContext.get_messages() truncates by repeatedly popping the message at len(messages) // 2 until the token budget is satisfied. The old cleanup only checked if messages[0] was a FunctionExecutionResultMessage. If truncation removed an AssistantMessage containing FunctionCalls while its paired FunctionExecutionResultMessage sat elsewhere in the list, the orphan was never repaired — leaving a tool result with no matching tool call.

Fix

Replace the index-0-only check with a full pass: collect all call_ids from remaining AssistantMessages with FunctionCalls, then filter out any FunctionExecutionResultMessages whose call_ids have no matching call.

Reproduction

With 7 messages and token_limit=6, exactly one pop is needed: middle_index = 7 // 2 = 3, which removes the AssistantMessage holding call_1. The old post-loop check only looks at index 0 (a UserMessage), so nothing gets fixed. The returned list still contains the orphaned FunctionExecutionResultMessage.

Closes #7955

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