Fix UI logical date timezone mismatch on DAG trigger

[Arunodoy18]

Problem

When triggering a DAG from the UI while the UI timezone is set to a non-UTC timezone, the logical date received by the backend does not match the logical date selected in the UI.

This results in inconsistent DAG run scheduling and incorrect logical date assignment.

---

Root Cause

Two issues were identified in the frontend:

1. TriggerDAGForm initialized logicalDate using a naive datetime format (YYYY-MM-DD HH:mm:ss).
   This produced timezone-ambiguous parsing behaviour when interpreted by browser Date() APIs.

2. DateTimeInput used a space separator instead of the T separator required by datetime-local inputs, causing inconsistent browser parsing behaviour.

---

Solution

• Use dayjs().toISOString() to ensure logical date is serialized as an unambiguous UTC datetime.
• Use YYYY-MM-DDTHH:mm:ss format for datetime-local display to comply with browser standards.
• Ensure frontend → API → backend logical date consistency.

---

Tests

Added tests to verify timezone correctness:

• Frontend logical date serialization tests
• Backend logical date parsing validation tests
• End-to-end logical date consistency validation

---

Impact

• Ensures consistent logical date handling across UI and backend.
• Prevents timezone-related scheduling mismatches when triggering DAGs.

---

closes: #61982

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

• Yes (ChatGPT, GitHub Copilot)

Generated-by: ChatGPT and GitHub Copilot following Airflow Gen-AI contribution guidelines