Add in-form "Select Recent Configurations" dropdown to the Trigger DAG form (navigation QoL, coexisting with "Trigger again with this config")

### Description

Airflow 3.2.0 restored config reuse via the "Trigger again with this config"
menu item (#56254 / #56406). That flow requires the user to first navigate to
the DAG run list/grid, visually find the right past run, select it, and only
then trigger from it.

This request is to ALSO add back the Airflow 2 "Select Recent Configurations"
dropdown *inside the trigger form itself*. Picking an entry pre-fills the
Configuration JSON without ever leaving the form.

The point of this request is the navigation quality-of-life, not config reuse in
the abstract. The two features are complementary:
- "Trigger again with this config": for when you already know and can find the
exact run you want to copy.
- In-form dropdown: for when you just want to grab a recent config while
composing a run, without breaking flow to go hunt through the run list.

Proposal: keep the menu item as-is and add the dropdown to the trigger form.
Both write the same conf field, so there is no conflict.

### Use case/motivation

Many DAGs are triggered manually with configs that repeat or differ by only a
few parameters. The friction is navigation, not the reuse itself.

Current 3.2.0 flow to reuse a config:
open DAG -> go to runs list/grid -> scan for the run with the config I want
-> select it -> Trigger button -> "Trigger again with this config" -> adjust
-> run.

When a DAG has run many times and runs look visually similar, finding "the run
that had the config I want" is slow and error-prone, and it pulls the user out
of the trigger flow entirely.

Airflow 2 flow (what this restores):
open trigger form -> pick a recent config from the dropdown -> adjust -> run.

The in-form dropdown removes the detour into the run list. It surfaces recent
configs right where the user is already composing the run. This is the specific
quality-of-life that the menu item does not provide, since the menu item is
built around first locating and selecting a specific run.

Multiple users have asked for exactly this (Discussion #56038). The dropdown and
the menu item can coexist with minimal added surface area, since the dropdown
just populates the existing conf field.

### Related issues

- #56254 Trigger form missing "Select Recent Configurations" from airflow 2 (implemented as the menu item)
- #56406 PR that added "Trigger again with this config" (3.2.0); a dropdown was built here but removed in favor of the menu item
- #56038 Discussion: Missing dropdown "Select Recent Configurations" in DagRun UI Airflow v3 (users still asking for this)
- #27714 Original Airflow 2 feature: Re-use recent DagRun JSON-configurations
- #36878 Airflow 2 PR: configurable number of recent configs in the dropdown

### Are you willing to submit a PR?

- [ ] 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)

Start at the Trigger DAG form entry point and inspect how the existing Configuration JSON field is populated. Trace the current "Trigger again with this config" flow and related recent-configuration behavior, then verify that selecting a recent entry pre-fills the form without removing the existing menu item.

Written by the indexing model from the issue text.