use_aws tool ignores AWS_PROFILE environment variable, defaults to 'default' profile

### Checks

- [x] I have searched [github.com/aws/amazon-q-developer-cli/issues](https://github.com/aws/amazon-q-developer-cli/issues?q=) and there are no duplicates of my issue

### Operating system

macOS 15.5.0 (24F74)

### Expected behaviour

The tool documentation states it uses profiles "from ~/.aws/credentials" and "defaults to default profile if not specified", but:

1. **SSO profiles are stored in ~/.aws/config, not ~/.aws/credentials** (this is correct AWS behavior)
2. **The tool should check AWS_PROFILE environment variable before defaulting to "default"**
3. **AWS SDK/CLI precedence should be: explicit parameter > environment variable > default profile**

## Workaround
Currently, the only workaround is to explicitly specify the profile in every `use_aws` call:
```
profile_name: ""
```

## Impact
- Breaks expected AWS CLI behavior consistency
- Forces users to manually specify profiles instead of using standard AWS environment variables
- Particularly problematic for SSO users who rely on environment variables for profile switching

## Proposed Solution
The `use_aws` tool should follow AWS CLI credential precedence:
1. Explicit `profile_name` parameter (highest priority)
2. `AWS_PROFILE` environment variable
3. "default" profile (lowest priority)

This would make the tool consistent with standard AWS CLI behavior and eliminate the need for explicit profile specification when environment variables are properly set.

## Additional Context
- This issue specifically affects SSO authentication workflows
- Standard AWS CLI commands work perfectly with the same configuration
- The problem is reproducible with any SSO-configured profile when using environment variables

### Actual behaviour

The use_aws tool ignores the AWS_PROFILE environment variable and always defaults to the "default" profile. Even with AWS_PROFILE=sandbox set in the environment, the tool shows "Profile name: default" instead of using the specified profile.

```

### Steps to reproduce

## Summary
The `use_aws` tool in Amazon Q Developer CLI ignores the `AWS_PROFILE` environment variable and defaults to the "default" profile, unlike the standard AWS CLI which properly respects environment variables.

## Environment
- **OS**: macOS (Darwin/24.5.0)
- **Amazon Q CLI Version**: q 1.12.1
- **AWS CLI Version**: aws-cli/2.27.40 Python/3.13.5 Darwin/24.5.0 source/arm64
- **Authentication Method**: AWS SSO (Single Sign-On)

## Current Behavior
When using AWS SSO authentication with `AWS_PROFILE=` environment variable set:

1. Standard AWS CLI commands work correctly:
```bash
$ export AWS_PROFILE=
$ aws sts get-caller-identity
# ✅ Uses specified profile correctly
```

2. Amazon Q CLI `use_aws` tool ignores the environment variable:
```
🛠️ Using tool: use_aws
● Running aws cli command:
Service name: sts
Operation name: get-caller-identity
Profile name: default # ❌ Should be ""
Region: us-east-1
```

## Expected Behavior
The `use_aws` tool should respect the `AWS_PROFILE` environment variable, just like the standard AWS CLI does.

## Configuration Details

### ~/.aws/config (SSO Configuration)
```ini
[profile management]
sso_session = aws-sso
sso_account_id =
sso_role_name = AWSAdministratorAccess
region =

[profile sandbox]
sso_session = aws-sso
sso_account_id =
sso_role_name = AWSPowerUserAccess
region =
output = json

[sso-session aws-sso]
sso_start_url = https://.awsapps.com/start
sso_region =
sso_registration_scopes = sso:account:access
```

### Authentication Workflow
```bash
# Login and set environment
aws sso login --profile
export AWS_PROFILE=
export AWS_DEFAULT_REGION=

# Verify standard AWS CLI works
aws sts get-caller-identity # ✅ Works with specified profile

# Launch Amazon Q CLI
q chat # ❌ use_aws tool defaults to "default" profile
```

## Root Cause Analysis
Based on the tool's function signature, the issue appears to be:

```
use_aws(
profile_name: str (optional), # "Defaults to default profile if not specified"
...
)
```