json_native.py: 14 surviving mutants under the repository's own committed mutmut scope

[cdeust]

What

[tool.mutmut] in pyproject.toml names mcp_server/shared/json_native.py as
the committed, "wired + demonstrated" mutation scope. Running exactly that
committed scope today leaves 14 surviving mutants, all in
to_json_native:

43/43  killed 29  no-tests 0  survived 14
mcp_server.shared.json_native.x_to_json_native__mutmut_11: survived
mcp_server.shared.json_native.x_to_json_native__mutmut_17: survived
mcp_server.shared.json_native.x_to_json_native__mutmut_21: survived
mcp_server.shared.json_native.x_to_json_native__mutmut_22: survived
mcp_server.shared.json_native.x_to_json_native__mutmut_23: survived
mcp_server.shared.json_native.x_to_json_native__mutmut_34..42: survived

Reproduce

uv run mutmut run          # committed [tool.mutmut] scope, no arguments
uv run mutmut results | grep -i survived

Measured 2026-07-29 on test-check-doc-claims-mutation-235
(mutmut 3.6.0, CPython 3.13.11, macOS). The branch's own diff does not touch
mcp_server/shared/; this was surfaced by an accidental unscoped invocation
while working #235 and is filed under §14.3 rather than fixed there, because
it is unreachable from that change by any test, build or lint step it
triggers.

Why it matters

to_json_native is the module that exists BECAUSE a numpy.float32/datetime
leak broke structuredContent on PostgreSQL while the SQLite-backed tests
stayed green (2026-06-23, quoted in [tool.mutmut]'s own comment). It is also
the module the standards file cites as the demonstrated mutation-testing
example. A third of its mutants surviving means its test suite does not pin
the conversions it was written to guarantee.

Acceptance criteria

• 0 surviving non-equivalent mutants under the committed [tool.mutmut]
  scope (uv run mutmut run with no arguments).
• Each survivor is triaged: killed by a named test, or documented as
  equivalent with a written rationale at the use site (§12.1).
• The kills assert the backend-agnostic contract (§12.3) — the numpy and
  datetime shapes PostgreSQL returns, not only the float/str SQLite
  gives — since that asymmetry is the incident this module exists for.