fix: repair CI lints and failing unit tests on master

[Liam Farrelly (lfarrel6)]

The Clippy and Unit Test jobs are red on master, independent of any
dependency bump. Three unrelated causes:

Clippy (stable is now 1.98, where unnecessary_unwrap also fires on
as_ref().unwrap(), plus useless_borrows_in_formatting and
useless_conversion hits):

• active_model_ex.rs / model_ex.rs: bind the LitStr with a pattern
  instead of re-checking is_some() and unwrapping.
• statement.rs, helper.rs, sea-orm-cli/commands/migrate.rs: drop the
  redundant borrows in write!/format! and the redundant into_iter().

Unit Test, cargo test --workspace failed to compile with
cannot find register_entity in sea_orm. DeriveEntity gates the registry
submission on sea-orm-macros' own entity-registry feature, but the
proc-macro crate is resolved once for the whole build while sea-orm is
not, so the macro can emit sea_orm::register_entity! into a sea-orm
built without the feature. sea-orm (and its sea-orm-sync mirror) now
provide a no-op register_entity! in that configuration, so the generated
code compiles either way.

cargo test --features entity-registry also failed to build the
integration tests: the submission named Entity as a value and hardcoded
that name, which breaks for an entity struct with fields, such as
tests/common/features/dyn_table_name.rs. Use the derived ident, and skip
entities that are not unit structs — those have no static schema to
register.

Finally, examples/quickstart goes back to being its own workspace.
As a member it unified sqlx-sqlite, runtime-tokio, schema-sync and
entity-registry into sea-orm's own test targets, which made the Unit
Test job compile and run the database integration tests, and those need a
DATABASE_URL that the job does not set.

Co-Authored-By: Claude Opus 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_013LqKqpvKd1hWCpWvAZeQmn

[Liam Farrelly (lfarrel6)]

Noticed CI was failing on an automated dependency patch for the JS example projects for unrelated Rust changes.