fix(tagstore): Skip array meta-columns in tag value paginator

[Zylphrex]

Summary

Fixes SENTRY-5PD7: TypeError: unhashable type: 'list'.

Querying tag values for the array meta-columns tags.key / tags.value crashed in
snuba.nest_groups. These columns map to themselves in the snuba column map and have no
arrayjoin (get_arrayjoin only matches exception_stacks|exception_frames|contexts), so the
backend issued a GROUP BY tags.key query and snuba returned an array per row. nest_groups
then used that list as a dict key → crash.

The trigger in production was the Seer public RPC get_event_filter_key_values, which forwarded
filter_key="tags.key" to the tags-values endpoint, but any caller of that endpoint was affected.

Fix

tags.key / tags.value are array meta-columns, not real tags, and have no meaningful values to
suggest. They now short-circuit to an empty paginator in get_tag_value_paginator_for_projects,
using the same early-return blacklist that already handles trace.*, event_id, timestamp, etc.

Testing

• Added test_get_tag_value_paginator_array_meta_columns asserting both keys return [] instead
  of raising.
• Verified the new test fails without the fix (reproduces the GROUP BY tags.key crash path) and
  passes with it.
• prek lint clean on both files.

Fixes SENTRY-5PD7