[FernandoCelmer]

Summary

• Gradual context truncation — removes largest context first, stops when under limit
• Fix max=0 falsy bug — not self.max → self.max is None
• JSON structured message — {"message": "Context size exceeded"} instead of plain string
• Remove size_message string sentinel, use dict field

Before vs After

Before: All 3 contexts replaced at once, even if only 1 is large

15MB payload → replace ALL contexts → clear errors → return

After: Contexts removed one at a time (largest first)

15MB payload → remove current_context (14MB) → 1MB → fits, return
                 ↑ initial and previous preserved

Changes

Issue	Fix
All-or-nothing truncation	Gradual: largest context first
max=0 skipped	self.max is None check
String sentinel "Context size exceeded"	JSON {"message": "Context size exceeded"}

Test plan

• test_with_max_truncates_largest_context_first — only largest removed
• test_with_max_truncates_all_contexts_if_needed — all removed when necessary
• test_with_max_zero_truncates — max=0 works
• test_with_max_clears_errors_if_still_over — errors cleared as last resort
• All 19 serializer tests pass
• ruff lint + format pass

Closes #252