Allow context gathering to exit early

[reytchison]

In seed-gen tasks, the context gathering step occurs a fixed number of times, e.g. for seed-explore it's specified here: https://github.com/trailofbits/buttercup/blob/993fa2241af5eb9b8532b9d5f36b942c5668f4ba/seed-gen/src/buttercup/seed_gen/seed_explore.py#L32

The agent sometimes stops making context gathering tool calls, e.g. because it's gathered all of the context it needs or because it thinks it found a vulnerability. The agent should be able to exit early instead of repeating the context gathering step. This will save tokens and speed up seed gen.