Performance: Eager load DagModel relationship on TaskInstance queries to avoid N+1 query storm

### Description

### Description

During the Airflow scheduling loop, when evaluating task instance scheduling decisions for active `DagRun`s (inside `DagRun.update_state()` -> `_get_ready_tis()`), the scheduler checks various task dependencies by calling `are_dependencies_met()`.

One of the standard checks is `DagUnpausedDep` which verifies if the parent DAG is paused or not. Previously, `DagUnpausedDep` performed an ad-hoc query directly on the database:
```python
return session.scalar(select(DagModel.is_paused).where(DagModel.dag_id == dag_id))
```
This triggers an **N+1 queries storm**. For every task instance being checked for execution, a separate database round-trip is made to check if its DAG is paused. On instances with hundreds of task instances, this results in hundreds of queries to the `dag` table in every scheduler tick.

### Proposed Solution
1. **Relationship Check**: Update `DagUnpausedDep` to check `ti.dag_model.is_paused` directly using the ORM relationship attribute.
2. **Eager Loading**: Update `DagRun.fetch_task_instances` (which is used by `DagRun.get_task_instances` inside `update_state`) to eager load the `dag_model` relationship using `joinedload(TI.dag_model)`.

This keeps the modular dependency check design, but reduces the database round-trips from $O(N)$ queries to $O(1)$ in-memory lookups.

### Benchmarking Results
We verified the query savings and performance gains on a mock dataset of **1,000 TaskInstances** across **100 distinct DAG Runs**:

* **Without Eager Loading (Lazy/Ad-Hoc)**: **101 queries** total, taking **610.05 ms**.
* **With Eager Loading (Joinedload)**: **1 query** total, taking **320.95 ms**.
* **Query Reduction**: **100 fewer database queries (99.01% query reduction)**.
* **Latency Speedup**: **1.90x faster** on SQLite.

### Affected Components
* `airflow-core/src/airflow/models/dagrun.py`
* `airflow-core/src/airflow/ti_deps/deps/dag_unpaused_dep.py`

### Use case/motivation

_No response_

### Related issues

_No response_

### Are you willing to submit a PR?

- [x] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

Read airflow-core/src/airflow/ti_deps/deps/dag_unpaused_dep.py and airflow-core/src/airflow/models/dagrun.py, starting at DagUnpausedDep and DagRun.fetch_task_instances as used by update_state. Confirm the relationship is available during _get_ready_tis() and that eager loading removes the per-task DAG query. Done means the dependency reads the relationship and the query count matches the proposed reduction.

Written by the indexing model from the issue text.