lastid plugin fix II - #4

Fixes #6601 

Added `import contextvars` and changed `_extend_pipeline` to capture the
current context and run the inner pipeline within it to ensure the
`music_dir` context variable is available when the inner pipeline
resolves paths, preventing the relative-path bug that caused could not
get filesize errors and removing 0 old duplicated items during merge.

Works perfectly after the fix:

```bash
arsaboo@arsmusic:~$ beet import -m -I -t ~/shared/music/ --set genre="Filmi" --search-id 7MwKD3kEFMov4LQqyhnmzL

/home/arsaboo/shared/music (1 items)

  Match (75.2%):
  King - Lukkhe
  ≠ artist, tracks
  Spotify, None, 2026, None, Warner Music India, None, None
  https://open.spotify.com/album/7MwKD3kEFMov4LQqyhnmzL
  ≠ Artist: King; OAFF; Savera; Sunny M.R. -> King
  * Album: Lukkhe
     ≠ (#2) Khamoshiyaan (3:03) -> (#2) Khamoshiyaan (feat. Romy & Manreet Khara) (3:03)
Missing tracks (13/14 - 92.9%):
 ! Bulletproof (#1) (3:09)
 ! Jee Lenge (#3) (3:25)
 ! Headshot (#4) (3:10)
 ! Ruh Teri (feat. Manreet Khara & Agrim Joshi) (#5) (2:31)
 ! Roobaroo (#6) (3:17)
 ! Savere (#7) (2:29)
 ! Haal (#8) (3:29)
 ! Haal (The Journey) (#9) (6:13)
 ! Nachdi Shaam (#10) (2:22)
 ! Hoga Bada Mera Naam (#11) (1:58)
 ! All Eyes On Us (#12) (3:13)
 ! Swan Song (Hoya Azaad) (#13) (3:17)
 ! Bhaari Pangey (#14) (2:19)
➜ [A]pply, More candidates, Skip, Use as-is, as Tracks, Group albums,
Enter search, enter Id, aBort, eDit, edit Candidates, Print tracks,
Open files with Picard? a
This album is already in the library!
Old: 3 items, MP3, 320kbps, 9:45, 22.9 MiB
New: 1 items, MP3, 320kbps, 3:03, 7.8 MiB
➜ [S]kip new, Keep all, Remove old, Merge all? m

/home/arsaboo/shared/music
/data/music/Hindi Music/L/Lukkhe [30718] (2026)/Lukkhe (2026) - Bulletproof.mp3
/data/music/Hindi Music/L/Lukkhe [30718] (2026)/Lukkhe (2026) - Headshot.mp3
/data/music/Hindi Music/L/Lukkhe [30718] (2026)/Lukkhe (2026) - Jee Lenge.mp3 (4 items)

  Match (82.6%):
  King - Lukkhe
  ≠ artist, tracks
  Spotify, None, 2026, None, Warner Music India, None, None
  https://open.spotify.com/album/7MwKD3kEFMov4LQqyhnmzL
  ≠ Artist: OAFF; Savera; Ruaa Kayy; Romy; Manreet Khara -> King
  * Album: Lukkhe
     * (#1) Bulletproof (3:09)
     ≠ (#2) Khamoshiyaan (3:03) -> (#2) Khamoshiyaan (feat. Romy & Manreet Khara) (3:03)
     * (#3) Jee Lenge (3:25)
     * (#4) Headshot (3:10)
Missing tracks (10/14 - 71.4%):
 ! Ruh Teri (feat. Manreet Khara & Agrim Joshi) (#5) (2:31)
 ! Roobaroo (#6) (3:17)
 ! Savere (#7) (2:29)
 ! Haal (#8) (3:29)
 ! Haal (The Journey) (#9) (6:13)
 ! Nachdi Shaam (#10) (2:22)
 ! Hoga Bada Mera Naam (#11) (1:58)
 ! All Eyes On Us (#12) (3:13)
 ! Swan Song (Hoya Azaad) (#13) (3:17)
 ! Bhaari Pangey (#14) (2:19)
➜ [A]pply, More candidates, Skip, Use as-is, as Tracks, Group albums,
Enter search, enter Id, aBort, eDit, edit Candidates, Print tracks,
Open files with Picard? a
```