adk web: asterisks in user messages rendered as Markdown italic instead of literal text

## 🔴 Required Information
*Please ensure all items in this section are completed to allow for efficient
triaging. Requests without complete information may be rejected / deprioritized.
If an item is not applicable to you - please mark it as N/A*

**Describe the Bug:**
The adk web chat UI interprets asterisk (*) characters in user messages as Markdown italic formatting instead of displaying them as literal text. Math expressions like `2*3*4*5 `are rendered as `234*5` in the chat bubble.

**Steps to Reproduce:**

1. Create any ADK agent
2. Run adk web
3. Open http://localhost:8000 and select the agent
4. Type `2*3*4*5` in the chat input and send
5. Observe the user message bubble renders as 234*5

**Expected Behavior:**
The user message bubble should display the literal text `2*3*4*5` exactly as typed.

**Observed Behavior:**
The asterisks are consumed as Markdown italic markers. `2*3*4*5` becomes 2 + 3 (italic) + 4 + 5 → displays as `234*5`. The agent receives the correct input (verified in the event panel on the left side), so this is purely a frontend rendering issue.

**Environment Details:**

- ADK Library Version (pip show google-adk): 1.27.1
- Desktop OS:** macOS
- Python Version (python -V): 3.12

**Model Information:**

- Are you using LiteLLM: No
- Which model is being used: gemini-2.5-flash

---

## 🟡 Optional Information
*Providing this information greatly speeds up the resolution process.*

**Regression:**
N/A (first time using adk web)

**Logs:**
N/A — no errors in logs, this is a frontend rendering issue only.

**Screenshots / Video:**
If applicable, add screenshots or screen recordings to help explain
your problem.

**Additional Context:**
Add any other context about the problem here.

**Minimal Reproduction Code:**
Please provide a code snippet or a link to a Gist/repo that isolates the issue.
```python
# Any agent will reproduce this. Minimal example:
from google.adk.agents import LlmAgent

root_agent = LlmAgent(
model="gemini-2.5-flash",
name="test_agent",
instruction="You are a helpful assistant.",
)
# Run: adk web
# Type: 2*3*4*5

```

**How often has this issue occurred?:**

- Always (100%)