* fix(memory): truncate mem0 context injection on entry boundaries
Mem0Manager.get_context built the injection block and then hard-cut it
at max_injection_chars, which could sever the last memory mid-line and
leave a dangling partial entry in the agent prompt.
Accumulate whole entries against the remaining budget instead: a
memory that does not fit is skipped (a shorter later one may still
fit). When not even the first memory fits, fall back to the previous
hard-truncation behavior for that single entry rather than injecting
nothing.
Tests: entry-boundary truncation, skip-oversized-keep-later, and the
oversized-first-entry fallback.
* fix(memory): keep entry-boundary guarantee when no mem0 memory fits
Follow-up to PR #4600 review: remove the hard-truncation fallback that
could inject a partial memory when max_injection_chars is smaller than
every recalled entry. Instead return empty context and log a warning that
surfaced the undersized budget.
