fix(memory): prevent oldest entry from dropping in STLTMemory

[Harshitmehra-270709]

Fixes #186

Describe the bug
STLTMemory._process_step_core() was previously prematurely popping the oldest entry from the short_term_memory queue before the LLM could summarize it via _update_long_term_memory(). This resulted in the oldest memory being completely lost.

The Solution

• Defer the popleft() call to occur exactly after the _update_long_term_memory() method calculates the new summary in both process_step and �process_step.
• Added a definitive regression test using the repro steps from the issue description to verify the behavior directly against the LLM mock call arguments.

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[IlamaranMagesh]

There are already PRs and issues opened on this. Please look into it #107