Fix lexer_callbacks silently dropped for keyword terminals (#1618)

[apoorvdarshan]

Summary

Fixes #1618. When two or more terminals each absorb a same-priority keyword literal (so they get an UnlessCallback) and have a lexer_callbacks entry, every user callback except the last silently never fires.

fired = []
p = Lark(r'''
    start: (A | B | AKW | BKW)+
    A: /[a-z]+/
    B: /[0-9]+/
    AKW: "foo"
    BKW: "123"
    %ignore " "
''', parser='lalr', lexer='basic',
    lexer_callbacks={'A': lambda t: fired.append('A') or t,
                     'B': lambda t: fired.append('B') or t})
list(p.lex("abc 456"))
# before: fired == ['B']   (A dropped)
# after:  fired == ['A', 'B']

Root cause

BasicLexer chains an existing UnlessCallback with the user callback:

self.callback[type_] = CallChain(self.callback[type_], f, lambda t: t.type == type_)

The lambda t: t.type == type_ closes over the loop variable type_, so after the loop every CallChain condition compares against the last terminal's name. For all but the last terminal the condition is never true, so the user callback is skipped.

Fix

Bind type_ per iteration via a default argument (lambda t, type_=type_: ...).

Verification

• Added TestGrammar.test_lexer_callbacks_with_keyword_terminals; it fails on master (only the last callback fires) and passes here.
• Full test suite: 1107 passed, 167 skipped (no regressions).
• Keyword absorption still works (the UnlessCallback continues to convert foo/123 to AKW/BKW, which correctly do not trigger the A/B callbacks).