enh: pivot_quantiles_longer adds quantile_level to epi_df other_keys

[dshemetov]

Checklist

Please:

• Make sure this PR is against "dev", not "main".
• Request a review from one of the current epipredict main reviewers:
  dajmcdon.
• Make sure to bump the version number in DESCRIPTION and NEWS.md.
  Always increment the patch version number (the third number), unless you are
  making a release PR from dev to main, in which case increment the minor
  version number (the second number).
• Describe changes made in NEWS.md, making sure breaking changes
  (backwards-incompatible changes to the documented interface) are noted.
  Collect the changes under the next release number (e.g. if you are on
  0.7.2, then write your changes under the 0.8 heading).
• Consider pinning the epiprocess version in the DESCRIPTION file if
  • You anticipate breaking changes in epiprocess soon
  • You want to co-develop features in epipredict and epiprocess

Change explanations for reviewer

Ran across this issue when writing YJ tests:

r$> # add quantile_level to epi_df other_keys, if epi_df
      tib <- tibble(geo_value = c("a", "b"), time_value = as.Date(c("2021-01-01", "202
    1-01-02")), d1 = d1, d2 = d2)

r$> epi_df <- tib %>% as_epi_df() %>% pivot_quantiles_longer(d1)

r$> epi_df
An `epi_df` object, 6 x 5 with metadata:
* geo_type  = custom
* time_type = day
* as_of     = 2025-04-10 15:11:56.915382

# A tibble: 6 × 5
  geo_value time_value        d2 d1_value d1_quantile_level
  <chr>     <date>     <qtls(3)>    <int>             <dbl>
1 a         2021-01-01     [2.5]        1              0.25
2 a         2021-01-01     [2.5]        2              0.5 
3 a         2021-01-01     [2.5]        3              0.75
4 b         2021-01-02     [3.5]        2              0.25
5 b         2021-01-02     [3.5]        3              0.5 
6 b         2021-01-02     [3.5]        4              0.75

Not a big deal, but probably good to strive for epi_df key uniqueness, rather than breaking the model.

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