fix(mypy): preserve complete diagnostics

[Agistaris]

Summary

• Preserve Mypy columns, severities, error codes, full messages, notes, native summaries, warnings, and fileless diagnostics.
• Keep deterministic ordering without truncating diagnostic text.
• Retain the shorter representation only when it remains fidelity-safe.

Test plan

• cargo fmt --all -- --check
• cargo clippy --all-targets
• Complete Cargo test matrix
• Manual Mypy 2.3.0 output comparison

The current focused command passes all 11 Mypy filter tests. The complete Cargo matrix passes. A real ephemeral Mypy 2.3.0 fixture preserved 24/24 diagnostics, every column, severity, code, complete message, note, native summary, and the nonzero exit status. The selected RTK representation reduced 3,037 characters to 2,886 characters (4.97%).

[CLAassistant]

All committers have signed the CLA.