fix: preserve MessageFilterAgent chronological order - #8075

Why are these changes needed?

MessageFilterAgent selected the correct messages for each source, but concatenated those selections in per_source configuration order. When source messages were interleaved, the wrapped agent therefore received a scrambled conversation timeline.

This change retains each selected message's original index and restores chronological order after all per-source filters have been applied. The existing first/last/count selection behavior remains unchanged.

A regression test reproduces the documented looping-agent case where a prior response from B must remain before A's newer revision.

Related issue number

Closes #7971

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. No documentation change is required because this restores the behavior described by the existing example.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

Test plan

• poe --directory packages/autogen-agentchat fmt (65 files unchanged)
• poe --directory packages/autogen-agentchat lint
• poe --directory packages/autogen-agentchat mypy (64 source files checked)
• poe --directory packages/autogen-agentchat pyright (0 errors)
• poe --directory packages/autogen-agentchat test (377 passed, 5 skipped)