Implicit API: local PropagateTags: false is silently overridden by global PropagateTags: true

### Describe the bug

`ImplicitApiPlugin._add_tags_to_implicit_api_if_necessary()` decides whether to copy a resource's `Tags` onto its generated implicit `AWS::Serverless::Api`/`HttpApi` resource using:

```python
should_propagate_tags = resource.properties.get("PropagateTags") or globals_var.get("PropagateTags")
```

`PropagateTags` is documented as `bool | None` (see `samtranslator/internal/schema_source/aws_serverless_function.py`), where `None` means "not set" and `False` is a meaningful, explicit opt-out. Using `or` means a local `PropagateTags: false` is indistinguishable from "not set" — it silently falls through to the global value.

### Reproduction

```yaml
Globals:
Function:
PropagateTags: true
Tags:
Team: Data

Resources:
MyFunction:
Type: AWS::Serverless::Function
Properties:
PropagateTags: false # explicit opt-out
Runtime: python3.12
Handler: index.handler
CodeUri: s3://bucket/key
Events:
Api:
Type: Api
Properties:
Path: /hello
Method: get
```

### Expected behavior

The function explicitly disables `PropagateTags`, so the generated implicit `AWS::Serverless::Api` should NOT receive the `Team: Data` tag.

### Actual behavior

`resource.properties.get("PropagateTags")` returns `False` (falsy), so `or` evaluates the global value (`True`) instead, and the tag is propagated to the implicit API anyway — silently ignoring the resource-level override.

This is inconsistent with the standard Globals merge logic (`GlobalProperties._prefer_local` in `samtranslator/plugins/globals/globals.py`), which always prefers the local value when the key is present locally, regardless of its truthiness.

### Fix

PR incoming.

The behavior is in ImplicitApiPlugin._add_tags_to_implicit_api_if_necessary(); read that entry point and compare its local/global selection with GlobalProperties._prefer_local in samtranslator/plugins/globals/globals.py. Done means an explicit false prevents tags from reaching generated implicit AWS::Serverless::Api or HttpApi resources, while an unset value still follows the global setting.

Written by the indexing model from the issue text.