fix(agents): validate phase transitions during exception recovery

[erensh27]

Fix for #238: the exception handlers in act() and observe() called the public set_phase() (which sets the phase without validating successors) instead of _set_phase() (which validates via check_successor), so error recovery could leave the agent in an invalid phase.

Changes in concordia/agents/entity_agent.py:

• Both except handlers now try _set_phase(Phase.READY) first, so recovery transitions are validated like the normal path.
• If the failure interrupted a transition (e.g. an exception during pre_act while the phase is PRE_ACT, where READY is not a valid successor), the validation itself would raise and mask the original exception. In that case we fall back to a direct set_phase(READY) reset so the entity stays usable for other threads.

No tests currently exist for EntityAgent (verified: tests/ and innertest coverage under concordia/agents/ is absent), so no regression test was added; the change is limited to the two recovery paths.

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[erensh27]

CLA check done

[jzleibo]

I believe we attempted to incorporate a change like this in the past, but some error handling path broke as a result, so we ended up reverting it. I'm not sure if it's exactly the same change or not. But I'm a bit reticent to change this part absent a very compelling reason to do so. If you have a specific situation where this change would help in a concrete way then we could take a look, but otherwise I'm inclined to leave this as it is for now.