fix: repair CI lints and failing unit tests on master - #28

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