sframe key api removal

[k-wasniowski]

Add key and epoch removal API

Summary

Adds remove_key and remove_epoch methods to Context and MLSContext respectively, allowing callers to remove individual keys or epochs without purging all prior state.

Changes

• Context::remove_key(KeyID) — Removes a single key from the SFrame context. Operations using a removed key return invalid_parameter_error. Removing a nonexistent key is a no-op.
• MLSContext::remove_epoch(EpochID) — Removes a single epoch and its derived keys, without affecting other epochs. Complements the existing purge_before() which removes all epochs below a threshold.
• Context::require_key(KeyID) — Protected helper that validates a key exists before use, returning an error Result instead of throwing via map::at. Used by protect, protect_inner, and unprotect_inner.
• map::erase(const K&) — Adds single-key erasure to the fixed-size map container.

Tests

• SFrame Context Remove Key — Verifies protect/unprotect fail after removal, and succeed again after re-adding the key.
• SFrame Context Remove Key - Nonexistent Key — Confirms removing a never-added key does not throw.
• MLS Remove Epoch — Verifies epoch-specific removal without affecting other epochs, and re-addition of a removed epoch.

[bifurcation]

One small comment, otherwise lgtm.