refactor(get,select,reject)!: deprecate --ignore-errors in favor of --optional (#16007)

# Description
As decided on the team meeting on 2025-06-19, rename `--ignore-errors
(-i)` to `--optional (-o)` with a (currently) indefinite grace period.

After `--ignore-errors (-i)` is removed, the short flag `-i` can be used
for `--ignore-case` (not implemented as of this PR)

# User-Facing Changes
`get`/`select`/`reject`: rename `--ignore-errors (-i)` to `--optional
(-o)` to better reflect its behavior.

# Tests + Formatting
- 🟢 toolkit fmt
- 🟢 toolkit clippy
- 🟢 toolkit test
- 🟢 toolkit test stdlib

# After Submitting
Update docs and inform third parties that integrate with nushell.

---------

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

crates/nu-command/tests/commands/default.rs

@@ -31,7 +31,7 @@ fn adds_row_data_if_column_missing() {
 
 #[test]
 fn default_after_empty_filter() {
-    let actual = nu!("[a b] | where $it == 'c' | get -i 0 | default 'd'");
+    let actual = nu!("[a b] | where $it == 'c' | get -o 0 | default 'd'");
 
     assert_eq!(actual.out, "d");
 }

crates/nu-command/tests/commands/get.rs

@@ -196,14 +196,14 @@ fn get_does_not_delve_too_deep_in_nested_lists() {
 
 #[test]
 fn ignore_errors_works() {
-    let actual = nu!(r#" let path = "foo"; {} | get -i $path | to nuon "#);
+    let actual = nu!(r#" let path = "foo"; {} | get -o $path | to nuon "#);
 
     assert_eq!(actual.out, "null");
 }
 
 #[test]
 fn ignore_multiple() {
-    let actual = nu!(r#"[[a];[b]] | get -i c d | to nuon"#);
+    let actual = nu!(r#"[[a];[b]] | get -o c d | to nuon"#);
 
     assert_eq!(actual.out, "[[null], [null]]");
 }

crates/nu-command/tests/commands/reject.rs

@@ -174,14 +174,14 @@ fn reject_multiple_rows_descending() {
 
 #[test]
 fn test_ignore_errors_flag() {
-    let actual = nu!("[[a, b]; [1, 2], [3, 4], [5, 6]] | reject 5 -i | to nuon");
+    let actual = nu!("[[a, b]; [1, 2], [3, 4], [5, 6]] | reject 5 -o | to nuon");
     assert_eq!(actual.out, "[[a, b]; [1, 2], [3, 4], [5, 6]]");
 }
 
 #[test]
 fn test_ignore_errors_flag_var() {
     let actual =
-        nu!("let arg = [5 c]; [[a, b]; [1, 2], [3, 4], [5, 6]] | reject ...$arg -i | to nuon");
+        nu!("let arg = [5 c]; [[a, b]; [1, 2], [3, 4], [5, 6]] | reject ...$arg -o | to nuon");
     assert_eq!(actual.out, "[[a, b]; [1, 2], [3, 4], [5, 6]]");
 }
 

crates/nu-command/tests/commands/select.rs

@@ -236,7 +236,7 @@ fn select_repeated_column() {
 fn ignore_errors_works() {
     let actual = nu!(r#"
         let path = "foo";
-        [{}] | select -i $path | to nuon
+        [{}] | select -o $path | to nuon
         "#);
 
     assert_eq!(actual.out, "[[foo]; [null]]");