fix(dc_measurements): constrain the Thermal Record's JSON schema

[Minipada]

Closes #300

thermal.json declared a type: object with additionalProperties: {type: number} and
nothing else — {} validated, and so did a Record whose only "zone" was keyed "". Every
other Measurement's schema says something about what it emits; this one didn't.

The shape stays; only the schema changed

Zone type strings are the field names (x86_pkg_temp, cpu-thermal, …), platform-specific
and unknowable ahead of time, so there is no fixed properties/required list to write — which
is why the original schema punted. Draft-07 has the vocabulary for this case:

• minProperties: 1 — at least one zone entry. For a map-shaped Record the entry is the
  field, so this is the "missing a required field" case. collect() already publishes nothing
  rather than a content-free {} when no zone can be read, so it can't fire on the plugin's own
  output: an empty Thermal Record reaching a Destination means something went wrong upstream.
• propertyNames: {minLength: 1} — a zone with an empty type file can't become a nameless
  key (readZone() already rejects one).
• additionalProperties: {type: number, minimum: -273.15} — degrees Celsius, above absolute
  zero, so a garbage temp read is caught rather than shipped.

base_path, zones, auto-discovery, the graceful-degradation contract and the emitted
{"<zone type>": <celsius>} map are untouched. With enable_validator off the schema is never
loaded (setValidationSchema() is already guarded on the flag), so behaviour there is unchanged.

Tests

3 → 8 in test_measurement_thermal.cpp. A ThermalSchema helper loads the installed
thermal.json via ament_index_cpp::get_package_share_directory() — the same file and the same
json_validator Measurement::validateJSON() uses, so the shipped artifact is what's exercised.

Test	Covers
AcceptsARepresentativeRecord	a two-zone Record validates
RejectsARecordWithNoZoneEntry	{} fails
RejectsMalformedZoneEntries	string temperature, sub-absolute-zero, empty zone key
PublishedRecordValidatesAgainstTheSchema	end-to-end via MeasurementServer, validator on
PublishesTheSameRecordWithTheValidatorOff	enable_validator: false, same Record

Worth knowing, since the first draft of the end-to-end test failed on it: enrichMsg() runs
validateJSON() first and only then adds name/nested/flattened/run_id/tags, so a
subscriber never sees the Record the validator saw. The test strips those framework keys — and
that ordering is why the schema can constrain values this tightly without breaking enrichment.

Verification

No colcon/ROS 2 in the dev sandbox, so this was built and tested for real in the cached
localhost/dc-workspace:latest Podman image with the worktree bind-mounted over
/root/ws/src/ros2_data_collection. colcon test --packages-select dc_measurements:
163 tests, 0 failures (8/8 thermal). The test log's INFO … schema: line confirms the new
schema is what the running plugin loads, and that Jazzy's nlohmann_json_schema_validator
implements propertyNames/minProperties — the one real unknown here. prek passes on all
changed files. doc/src/dc/measurements/thermal.md carries the new schema and a sentence on why
it's shaped this way rather than as a required list.

🤖 Generated with Claude Code

https://claude.ai/code/session_01BNt4fpZwLcgztWNjcLxAKf

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.54%. Comparing base (7062dfa) to head (a08311d).

Additional details and impacted files

@@           Coverage Diff           @@
##            jazzy     #351   +/-   ##
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  Coverage   67.54%   67.54%           
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  Files          95       95           
  Lines        5884     5884           
=======================================
  Hits         3974     3974           
  Misses       1910     1910           

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