to <format>: preserve round float numbers' type (#16016)

- fixes #16011

# Description
`Display` implementation for `f64` omits the decimal part for round
numbers, and by using it we did the same.
This affected:
- conversions to delimited formats: `csv`, `tsv`
- textual formats: `html`, `md`, `text`
- pretty printed `json` (`--raw` was unaffected)
- how single float values are displayed in the REPL

> [!TIP]
> This PR fixes our existing json pretty printing implementation.
> We can likely switch to using serde_json's impl using its
PrettyFormatter which allows arbitrary indent strings.

# User-Facing Changes
- Round trips through `csv`, `tsv`, and `json` preserve the type of
round floats.
- It's always clear whether a number is an integer or a float in the
REPL
  ```nushell
  4 / 2
  # => 2  # before: is this an int or a float?

  4 / 2
  # => 2.0  # after: clearly a float
  ``` 

# Tests + Formatting
Adjusted tests for the new behavior.

- 🟢 toolkit fmt
- 🟢 toolkit clippy
- 🟢 toolkit test
- 🟢 toolkit test stdlib

# After Submitting
N/A

---------

Co-authored-by: Bahex <17417311+Bahex@users.noreply.github.com>

crates/nu-command/tests/commands/math/avg.rs

@@ -38,5 +38,5 @@ fn cannot_average_infinite_range() {
 #[test]
 fn const_avg() {
     let actual = nu!("const AVG = [1 3 5] | math avg; $AVG");
-    assert_eq!(actual.out, "3");
+    assert_eq!(actual.out, "3.0");
 }

crates/nu-command/tests/commands/math/log.rs

@@ -3,7 +3,7 @@ use nu_test_support::nu;
 #[test]
 fn const_log() {
     let actual = nu!("const LOG = 16 | math log 2; $LOG");
-    assert_eq!(actual.out, "4");
+    assert_eq!(actual.out, "4.0");
 }
 
 #[test]

crates/nu-command/tests/commands/math/mod.rs

@@ -101,7 +101,7 @@ fn division_of_ints() {
         "#
     ));
 
-    assert_eq!(actual.out, "2");
+    assert_eq!(actual.out, "2.0");
 }
 
 #[test]
@@ -189,7 +189,7 @@ fn floor_division_of_floats() {
         "#
     ));
 
-    assert_eq!(actual.out, "-2");
+    assert_eq!(actual.out, "-2.0");
 }
 
 #[test]

crates/nu-command/tests/commands/math/round.rs

@@ -18,14 +18,14 @@ fn can_round_very_large_numbers_with_precision() {
 fn can_round_integer_with_negative_precision() {
     let actual = nu!("123 | math round --precision -1");
 
-    assert_eq!(actual.out, "120")
+    assert_eq!(actual.out, "120.0")
 }
 
 #[test]
 fn can_round_float_with_negative_precision() {
     let actual = nu!("123.3 | math round --precision -1");
 
-    assert_eq!(actual.out, "120")
+    assert_eq!(actual.out, "120.0")
 }
 
 #[test]

crates/nu-command/tests/commands/math/sqrt.rs

@@ -18,13 +18,13 @@ fn can_sqrt_irrational() {
 fn can_sqrt_perfect_square() {
     let actual = nu!("echo 4 | math sqrt");
 
-    assert_eq!(actual.out, "2");
+    assert_eq!(actual.out, "2.0");
 }
 
 #[test]
 fn const_sqrt() {
     let actual = nu!("const SQRT = 4 | math sqrt; $SQRT");
-    assert_eq!(actual.out, "2");
+    assert_eq!(actual.out, "2.0");
 }
 
 #[test]

crates/nu-command/tests/commands/math/variance.rs

@@ -3,7 +3,7 @@ use nu_test_support::nu;
 #[test]
 fn const_variance() {
     let actual = nu!("const VAR = [1 2 3 4 5] | math variance; $VAR");
-    assert_eq!(actual.out, "2");
+    assert_eq!(actual.out, "2.0");
 }
 
 #[test]