Add a tool-failure shape-mutation operator (nested / alternate-key / absent)

[AshwinUgale]

Context

Surfaced by a commenter on the muteval↔tracelint dev.to post. deny_tool_output emits one fixed shape ({"status": "declined"}) keyed to /status, so pairing it with a failure_when on /status proves the rule matches the shape the mutant emits, not the shapes real providers emit — a fair critique. The next operator should mutate the failure's representation, not its value, to test the robustness of the failure contract itself.

What to add

A shape-mutation operator (or small family) over tool outputs:

• nested code: {"outcome": {"code": "card_declined"}}
• alternate key / value scheme
• absent — a 200 with the failure field dropped entirely. This deserves its own operator: it's the mutant that reveals a contract which passes vacuously on a missing field.

Why absent matters

A failure_when/eval contract that keys on /status can't separate not-declined from not-present, so a dropped field passes clean — a green suite for exactly the regression the contract was written to catch. This is confirmed on the tracelint side (a declared failure_when whose pointer is absent currently passes clean instead of failing closed — filed separately in AshwinUgale/tracelint). The absent mutant is the muteval-side probe that surfaces that class of hole in a user's suite.

Acceptance criteria

• New operator(s) mutating a tool output's failure representation (nested, alternate-key, absent); one mutant per tool output per shape; target="tools".
• absent removes the declared failure field (or emits an unrelated-shape body).
• Registered in OPERATORS (near deny_tool_output, src/muteval/mutators.py:623) and src/muteval/severity.py (HIGH — same class as deny/corrupt).
• Tests per operator.

Cross-reference

Two halves of one story: muteval injects the absent shape; tracelint must not pass it vacuously. Link the tracelint issue once filed.