fix: Fix translation of quantile() to use DuckDB's native QUANTILE_CONT() syntax

[Copilot]

The dbplyr quantile() translation generates PERCENTILE_CONT(probs) WITHIN GROUP (ORDER BY x) OVER () which DuckDB doesn't support as a window function.

library(dbplyr)
library(dplyr)

con <- DBI::dbConnect(duckdb::duckdb())
mtcars_db <- copy_to(con, mtcars)

# Previously failed with: "Aggregate Function with name percentile_cont does not exist!"
mutate(mtcars_db, mpg_25th = quantile(mpg, 0.25, na.rm = TRUE))

Changes

• Added custom quantile aggregate translation using QUANTILE_CONT(x, probs) syntax
• Added custom quantile window translation with proper win_over() wrapping
• Added QUANTILE_CONT to globalVariables for R CMD check
• Added translation tests for aggregate and window contexts

Warning

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I tried to connect to the following addresses, but was blocked by firewall rules:

• cloud.r-project.org
  • Triggering command: /opt/R/4.5.2/lib/R/bin/exec/R /opt/R/4.5.2/lib/R/bin/exec/R -q -e options(repos~+~=~+~c(CRAN='REDACTED'));~+~install.packages(c('DBI',~+~'dbplyr',~+~'dplyr',~+~'testthat',~+~'rlang',~+~'cli',~+~'withr',~+~'brio')) (dns block)
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• cran.r-project.org
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• cran.rstudio.com
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• esm.ubuntu.com
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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Incorrect quantile translation</issue_title>
<issue_description>``` r
library(dbplyr)
library(dplyr, warn.conflicts = FALSE)

con <- DBI::dbConnect(duckdb::duckdb())
mtcars_db <- copy_to(con, mtcars)

mutate(mtcars_db, mpg_25th = quantile(mpg, 0.25, na.rm = TRUE))
#> Error in collect():
#> ! Failed to collect lazy table.
#> Caused by error in dbSendQuery():
#> ! Catalog Error: Aggregate Function with name percentile_cont does not exist!
#> Did you mean "pi"?
#>
#> LINE 3: PERCENTILE_CONT(0.25) WITHIN GROUP (ORDER BY mpg) OVER ...
#> ^
#> ℹ Context: rapi_prepare
#> ℹ Error type: CATALOG
#> ℹ Raw message: Aggregate Function with name percentile_cont does not exist!
#> Did you mean "pi"?
#>
#> LINE 3: PERCENTILE_CONT(0.25) WITHIN GROUP (ORDER BY mpg) OVER ...
#> ^

<sup>Created on 2025-11-26 with [reprex v2.1.1](https://reprex.tidyverse.org)</sup>

A little unfortunate we get two errors too.</issue_description>

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

<comments>
</comments>

• Fixes Incorrect quantile translation #1734

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