Use explicit namespace qualification in tally_n()

[Copilot]

Use explicit namespace qualification (dplyr::n() and base::sum()) instead of bare function names in quosure expressions to satisfy duckplyr's function identity checks.

Changes

• Changed expr(n()) to expr(dplyr::n()) in the null weight case
• Changed expr(sum(!!wt, na.rm = TRUE)) to expr(base::sum(!!wt, na.rm = TRUE)) in the weighted case
• Reverted environment from baseenv() back to quo_get_env(wt) for weighted case since explicit qualification makes environment less critical

This approach is cleaner than using quosures with specific environments because the namespace qualification is directly in the expression.

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