dbt integration: Add validation for timestamp field data type

## Context
PR #5827 added dbt integration that requires a timestamp field for point-in-time correctness.

## Problem
The code validates that the timestamp field exists in the model columns but doesn't validate that it's actually a timestamp type. This could lead to runtime errors or incorrect behavior.

## Current Behavior
```python
# In dbt_import.py:183-189
if timestamp_field not in column_names:
click.echo(
f"{Fore.YELLOW}Warning: Model '{model.name}' missing timestamp "
f"field '{timestamp_field}'. Skipping.{Style.RESET_ALL}"
)
continue
```

No type checking is performed.

## Proposed Solution
Add validation that the timestamp field is one of the timestamp types:
- TIMESTAMP
- TIMESTAMP_NTZ / TIMESTAMP_LTZ / TIMESTAMP_TZ
- DATETIME
- DATE

Warn users if the field exists but has an incompatible type.

## Example
```python
timestamp_col = next((c for c in model.columns if c.name == timestamp_field), None)
if timestamp_col:
normalized_type = timestamp_col.data_type.upper()
valid_ts_types = ['TIMESTAMP', 'TIMESTAMP_NTZ', 'TIMESTAMP_LTZ', 'TIMESTAMP_TZ', 'DATETIME', 'DATE']
if not any(t in normalized_type for t in valid_ts_types):
click.echo(f"{Fore.YELLOW}Warning: Timestamp field '{timestamp_field}' has type '{timestamp_col.data_type}' which may not be suitable{Style.RESET_ALL}")
```

## Related
- PR #5827