Harness: expose botocore read_timeout for bedrock-runtime model calls

## Problem

The AgentCore Harness runtime internally constructs a `BedrockModel` (Strands SDK) without exposing the `boto_client_config` parameter to customers. The default `read_timeout` of 120 seconds is insufficient for agents with complex tool chains where accumulated context causes time-to-first-token (TTFT) to exceed 120s.

## Impact

- **100% failure rate** on our security reviewer agent (Sonnet 4.5, 8 gateway tools)
- 56 timeout events across 7 invocations in 24 hours
- Any agent requiring >3-5 tool call cycles is at risk
- Model confirmed via OTel metrics: `global.anthropic.claude-sonnet-4-5-20250929-v1:0`
- Even Sonnet 4.5 (not just Opus) times out when context grows from accumulated tool results

## Root Cause

From `strands/models/bedrock.py`:
```python
DEFAULT_READ_TIMEOUT = 120

# If boto_client_config is NOT passed (harness default path):
client_config = BotocoreConfig(user_agent_extra="strands-agents", read_timeout=DEFAULT_READ_TIMEOUT)
```

The Harness constructs `BedrockModel` without customer-controlled `boto_client_config`, so there's no way to increase `read_timeout`.

## Stack Trace (from CloudWatch)

```
strands/event_loop/event_loop.py:238 → event_loop_cycle
strands/event_loop/event_loop.py:646 → _handle_tool_execution
strands/event_loop/event_loop.py:318 → recurse_event_loop
strands/event_loop/event_loop.py:198 → event_loop_cycle (model invocation)
→ botocore eventstream → urllib3 ReadTimeoutError

urllib3.exceptions.ReadTimeoutError: AWSHTTPSConnectionPool(host='bedrock-runtime.us-east-1.amazonaws.com', port=443): Read timed out.
```

## Proposed Solution

Expose a `model_read_timeout` (or full `boto_client_config`) parameter in one or more of:

1. `harness.json` configuration file
2. `InvokeHarness` API / `agentcore invoke` CLI
3. Environment variable (e.g., `BEDROCK_READ_TIMEOUT_SECONDS`)

## Workaround (requires ditching Harness)

Deploy as a regular Strands agent on AgentCore Runtime and pass `boto_client_config` directly:

```python
from botocore.config import Config as BotocoreConfig
from strands.models import BedrockModel

model = BedrockModel(
model_id="us.anthropic.claude-sonnet-4-5-20250929-v1:0",
boto_client_config=BotocoreConfig(read_timeout=300)
)
```

This defeats the purpose of the managed Harness experience.

## Environment

- Account: 023392223961
- Region: us-east-1
- Runtime: `harness_team_security_reviewer-Lpq9zN5Ve7`
- Log Group: `/aws/bedrock-agentcore/runtimes/harness_team_security_reviewer-Lpq9zN5Ve7-DEFAULT`
- Trace ID: `6a0c0285679e59e00d1010a7ee2a910e`
- Strands SDK version: bundled with Harness runtime (OTel SDK 1.40.0, auto-instrumentation 0.17.0-aws)