fix: report evicted handles as "evicted", not "not found" (#532)

[Shashankss1205]

Fixes #532.

Problem

Both handle stores silently bulk-evict the oldest entries at their cap (estimator store: 10 oldest via HandleManager._cleanup_oldest; data store: oldest ~20% via Executor._cleanup_oldest_data), emitting only a logger.debug. The caller then got a generic "not found" for a handle it had legitimately created — indistinguishable from a typo. (Confirmed live: a session-start data handle vanished after ~52 loads with a plain not-found.)

Fix

Track evicted ids in a bounded tombstone deque(maxlen=1024) in both stores, and route every not-found site through a shared helper:

• HandleManager.describe_missing(id) — used by get_instance/get_info (raise), evaluate, export_code, save_model, release_handle.
• Executor.data_handle_missing(id) — returns the error string + capped available-handles summary, used by inspect_data, split_data, save_data, transform_data, plot_series, and the executor's own format_data_handle / release_data_handle.

When the id was evicted, the message becomes "'' was evicted (handle limit N reached); reload the source / re-create it with instantiate" instead of "not found". Eviction is now logged at info level.

Testing

• New tests/test_eviction_tombstones.py (4 tests): estimator + data eviction messages, get_instance raises the eviction message, and inspect_data surfaces it end-to-end; a never-seen id still reads as a plain "not found".
• Full suite: 254 passed.

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