Reject team names that differ only in case

[potiuk]

Stacked on #70884 — both
touch team_command.py. Review that one first; this PR's own change is the
second commit.

Team names are case sensitive — data_eng and Data_Eng are two distinct teams,
both accepted by ^[a-zA-Z0-9_-]{3,50}$ — but the environment secrets backend
upper-cases the team name when it builds the variable name:

os.environ.get(f"{CONN_ENV_PREFIX}_{team_name.upper()}___" + conn_id.upper())

So both resolve AIRFLOW_CONN__DATA_ENG___<ID>. The two teams share one secrets
namespace: each reads the other's Connections and Variables, and whichever
provisions a variable last wins.

Whether the two rows can coexist at all is decided by the database's collation
on a String primary key rather than by Airflow, so the same configuration
behaves differently across backends — on a case sensitive collation both persist
and the namespaces merge; on a case insensitive one the second teams create
fails on the primary key.

Approach

Reject the collision at creation, on both paths.

• teams create refuses a name that upper-cases onto an existing team's name.
• teams sync checks the incoming bundle names against each other as well as
  against the stored ones, because one bundle config can introduce both halves of
  a collision in the same run.

The alternative — dropping the upper() fold so team identity stays case
sensitive end to end — was rejected: environment variable names are conventionally
upper-case, and it would silently change which variable an existing deployment
resolves. Failing at the moment the name is chosen is the safer half of that
trade, and it makes behaviour uniform across database collations.

upper() rather than casefold() is deliberate: it is exactly what the backend
applies, and team names are ASCII-only by the name pattern.

Behaviour change

airflow teams create and airflow teams sync now fail with an explanatory
message when a name differs from an existing team's only by case. Deployments that
already have such a pair are not migrated by this change — the collision
already exists there and is reported by neither command. Names differing by more
than case are unaffected.

Test plan

• test_team_command.py — 23 cases pass
• test_team_create_rejects_a_name_differing_only_in_case — creating data_eng
  then Data_Eng fails, and no second row is written
• test_team_create_allows_a_name_differing_by_more_than_case — data_eng and
  data_eng2 coexist
• ruff check / ruff format clean

Was generative AI tooling used to co-author this PR?

• Yes — Claude Opus 5 (1M context)

Generated-by: Claude Opus 5 (1M context) following the guidelines at
https://github.com/apache/airflow/blob/main/contributing-docs/05_pull_requests.rst#gen-ai-assisted-contributions