Support using timesnaps without quotes

Signed-off-by: Yuanchun Shen <yuanchu@amazon.com>

Author: yuancu

docs/user/ppl/cmd/search.rst

@@ -72,7 +72,7 @@ Read more details on time modifiers `here <https://github.com/opensearch-project
 **Notes:**
 
 * **Column name conflicts**: If your data contains columns named "earliest" or "latest", use backticks to access them as regular fields (e.g., ```earliest`="value"``) to avoid conflicts with time modifier syntax.
-* **Time snap syntax**: Time modifiers with time snaps must be wrapped in quotes (e.g., ``latest='+1d@d'``) for proper query parsing.
+* **Time snap syntax**: Time modifiers with chained time snaps must be wrapped in quotes (e.g., ``latest='+1d@month-10h@day'``) for proper query parsing.
 
 Default Field Configuration
 ===========================

integ-test/src/test/java/org/opensearch/sql/ppl/SearchCommandIT.java

@@ -952,7 +952,7 @@ public void testSearchWithChainedRelativeTimeRange() throws IOException {
     JSONObject result1 =
         executeQuery(
             String.format(
-                "search source=%s earliest='2025-08-01 03:47:41' latest='+10months@year' | fields"
+                "search source=%s earliest='2025-08-01 03:47:41' latest=+10months@year | fields"
                     + " @timestamp",
                 TEST_INDEX_TIME_DATA));
     verifySchema(result1, schema("@timestamp", "timestamp"));

ppl/src/main/antlr/OpenSearchPPLLexer.g4

@@ -227,7 +227,7 @@ SINGLE_QUOTE:                       '\'';
 DOUBLE_QUOTE:                       '"';
 BACKTICK:                           '`';
 ARROW:                              '->';
-AT:                                 '@';
+fragment AT:                        '@';
 
 // Operators. Bit
 
@@ -534,6 +534,15 @@ PERCENTILE_SHORTCUT:                PERC(INTEGER_LITERAL | DECIMAL_LITERAL) | 'P
 
 // LITERALS AND VALUES
 //STRING_LITERAL:                     DQUOTA_STRING | SQUOTA_STRING | BQUOTA_STRING;
+fragment TIME_SNAP_UNIT:              S | SEC | SECOND
+                                    | M | MIN | MINUTE
+                                    | H | HR | HOUR | HOURS
+                                    | D | DAY
+                                    | W | WEEK
+                                    | MON | MONTH
+                                    | Q | QTR | QUARTER
+                                    | Y | YR | YEAR;
+TIME_SNAP:                          AT TIME_SNAP_UNIT;
 ID:                                 ID_LITERAL;
 CLUSTER:                            CLUSTER_PREFIX_LITERAL;
 INTEGER_LITERAL:                    DEC_DIGIT+;

ppl/src/main/antlr/OpenSearchPPLParser.g4

@@ -772,13 +772,10 @@ timeModifierValue
    | DECIMAL_LITERAL
    | INTEGER_LITERAL
    | stringLiteral
-   | timeSnap
-   | (PLUS | MINUS) (integerLiteral)? timeModifierUnit (timeSnap)?
+   | TIME_SNAP
+   | (PLUS | MINUS) (integerLiteral)? timeModifierUnit (TIME_SNAP)?
    ;
 
-timeSnap
-  : AT timeModifierUnit;
-
 // tables
 tableSource
    : tableQualifiedName
@@ -1360,6 +1357,7 @@ timespanUnit
    | DS
    ;
 
+// It differs from TIME_SNAP_UNIT in that it supports plural forms
 timeModifierUnit
    : S
    | SEC