Add support for "After The Weekend" and "Before The Weekend" expressions

[Copilot]

Two new relative date expressions were not recognized by the parser:

• After The Weekend → the Monday following the upcoming weekend
• Before The Weekend → the Friday immediately before the upcoming weekend

Grammar (DateParser.g)

Added two alternatives to the relative_date rule. The lexer already maps "weekend" → SATURDAY, so these phrases tokenize naturally:

// after the weekend -> next monday
| AFTER WHITE_SPACE THE WHITE_SPACE SATURDAY
    -> ^(RELATIVE_DATE ^(SEEK DIRECTION[">"] SEEK_BY["by_day"] INT["1"] ^(DAY_OF_WEEK INT["2"])))

// before the weekend -> next friday
| BEFORE WHITE_SPACE THE WHITE_SPACE SATURDAY
    -> ^(RELATIVE_DATE ^(SEEK DIRECTION[">"] SEEK_BY["by_day"] INT["1"] ^(DAY_OF_WEEK INT["6"])))

Both expressions are case-insensitive via the existing ANTLRNoCaseInputStream. No changes to the walker or WalkerState were needed — the produced AST nodes reuse the existing seekToDayOfWeek path.

Tests (DateTest.java)

Added four cases against the existing reference date of Feb 28, 2011 (Monday), where the upcoming weekend is Mar 5–6:

Expression	Result
"after the weekend" / "After The Weekend"	March 7, 2011
"before the weekend" / "Before The Weekend"	March 4, 2011

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Feature] Add support for After/Before The Weekend</issue_title>
<issue_description>Add support for the following expressions:

• After The Weekend: Return the next Monday that comes after the upcoming weekend.
• Before The Weekend: Return the Friday immediately before the upcoming weekend.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Feature] Add support for After/Before The Weekend #135

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