The session-info modal showed "817 288 / 131 072 (100%)" for a session whose
context held a 1.47 MB code_exec result. The estimate was honest about
session.context, but build() head/tail-truncates every oversized tool/assistant
message to context_message_token_budget before the call, so the model never
received that megabyte.

truncate_oversized() moves the per-build rule to module level (the method now
delegates, behaviour unchanged) so the compressor can price the same view it
will send: compress_and_save_session stores estimate_tokens(truncate_oversized(
new_context)) instead of estimate_tokens(new_context).

Full content stays in stored history — the user still sees it, reloads are
unaffected. Regression test keeps a 1 MB tool result in the context and asserts
the stored count equals the truncated-view estimate and is >10x smaller than
the raw one.