Agent V2 `request_limit` is hardcoded to 50 with no way to configure

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### 1. Is this request related to a challenge you're experiencing? Tell me about your story.

In Dify 1.16.0 / 1.16.1, the Agent V2 node uses pydantic-ai's `UsageLimits()` which defaults to `request_limit=50`. This means any Agent V2 run that exceeds 50 LLM calls fails with:

```
pydantic_ai.exceptions.UsageLimitExceeded: The next request would exceed the request_limit of 50
```

There is **no way to adjust this value** — not in the UI, not via env vars, not via API parameters.

### Comparison with Agent V1

Agent V1 has `max_iteration` which is:
✅ Configurable in the UI (node settings panel)
✅ Defaults to 10, adjustable up to any value
✅ Stored in `app_config.agent_mode.max_iteration`

Agent V2 has `request_limit`:
❌ Hardcoded to 50 in pydantic-ai library
❌ No UI control
❌ No env var override
❌ No API parameter
### Impact

- Complex Agent workflows that need tool calling + reasoning easily exceed 50 calls
- Users are forced to decompose workflows into smaller agents, which contradicts the "single agent with tools" paradigm
- Community Edition users have no workaround (monkey-patching a third-party library is not acceptable for production)

### Request

Expose `request_limit` (or equivalent) as a configurable parameter in the Agent V2 node settings panel, with a reasonable maximum (e.g. 500).

This should be available in **all editions** (Community / Cloud / Enterprise).

### Additional context

- Dify version: 1.16.1
- The `request_limit` comes from pydantic-ai's `UsageLimits` class: `UsageLimits(request_limit=50)`
- The Agent V2 node creates pydantic-ai agents via `agenton_collections/layers/pydantic_ai/bridge.py`
- Agent V1 `max_iteration` is handled in `web/app/components/app/configuration/config/agent/agent-setting/index.tsx`
- No equivalent setting exists for Agent V2 in `web/features/agent-v2/`

### 2. Additional context or comments

_No response_

### 3. Can you help us with this feature?

- [x] I am interested in contributing to this feature.

Start in agenton_collections/layers/pydantic_ai/bridge.py to trace how UsageLimits is created, then compare the Agent V1 setting in web/app/components/app/configuration/config/agent/agent-setting/index.tsx with the Agent V2 area under web/features/agent-v2/. Done means Agent V2 exposes a request limit in node settings, supports a maximum such as 500, and works across Community, Cloud, and Enterprise editions.

Written by the indexing model from the issue text.