fix(json): preserve floats with trailing zero when encoding YAML to JSON (#2701)

YAML scalars tagged `!!float` were round-tripped through `float64` and
re-serialized by Go's JSON encoder, which strips the decimal part of
whole-number floats. As a result, `50.0` came out as `50` and a
sequence like `[50.0, 95.0, 99.0, 99.9]` became `[50,95,99,99.9]`,
turning a uniform array of floats into a mixed int/float array that
downstream consumers (Horreum, JSON Schema validators, jq, etc.)
reject.

The JSON spec does not distinguish ints from floats, but every common
JSON library (Go's `encoding/json`, Python's `json`, jq) preserves the
fractional form of values that came in as floats. yq's YAML decoder
already parses these as `!!float` with the original text intact, so we
can emit them verbatim instead of round-tripping.

`MarshalJSON` for `ScalarNode` now special-cases `!!float`:
- if `Value` is already a JSON-shaped number literal containing a `.`
  or exponent, emit it verbatim (e.g. `50.0`, `99.9`, `1.5e-3`, `-7.0`);
- if `Value` is an integer-shaped string tagged `!!float` (e.g.
  `!!float 5`), format the parsed float and append `.0` so it stays a
  JSON number with a fractional part;
- otherwise (empty value, parse error, or non-finite result), fall back
  to the existing encoding path so behaviour for `.inf` / `.nan` and
  anything unusual is unchanged.

`!!int` nodes still encode as JSON integers.

Closes #2683

Signed-off-by: ChrisJr404 <chris@hacknow.com>

pkg/yqlib/doc/usage/convert.md

@@ -125,6 +125,22 @@ will output
 {"whatever":"cat"}
 ```
 
+## Encode json: preserve floats with trailing zero
+Whole-number floats keep their decimal point so downstream consumers see a JSON number with a fractional part (matches Go's encoding/json, Python's json, and jq).
+
+Given a sample.yml file of:
+```yaml
+percentiles: [50.0, 95.0, 99.0, 99.9]
+```
+then
+```bash
+yq -o=json -I=0 '.' sample.yml
+```
+will output
+```json
+{"percentiles":[50.0,95.0,99.0,99.9]}
+```
+
 ## Roundtrip JSON Lines / NDJSON
 Given a sample.json file of:
 ```json