Bug: file_based_stream_reader.filter_files_by_globs_and_start_date uses strict datetime parsing, rejecting valid ISO8601 dates

## Description

`AbstractFileBasedStreamReader.filter_files_by_globs_and_start_date()` in `airbyte_cdk/sources/file_based/file_based_stream_reader.py` (line 108) uses `datetime.strptime(self.config.start_date, self.DATE_TIME_FORMAT)` where `DATE_TIME_FORMAT = "%Y-%m-%dT%H:%M:%S.%fZ"`.

This strictly requires microsecond digits in the `start_date` value. A valid ISO8601 date like `2025-01-01T00:00:00Z` (without microseconds) is rejected with:

```
ValueError: time data '2025-01-01T00:00:00Z' does not match format '%Y-%m-%dT%H:%M:%S.%fZ'
```

## Context

CDK v7.7.1 (via `airbytehq/airbyte-python-cdk` PR 887) fixed the **spec validation** side of this issue by:
1. Updating the JSON Schema `start_date` pattern to accept flexible date formats
2. Updating the Pydantic validator to use `ab_datetime_try_parse`

However, the **runtime** code path in `filter_files_by_globs_and_start_date` was not updated and still uses the strict `datetime.strptime` with `%Y-%m-%dT%H:%M:%S.%fZ`. This means config validation passes but the connector fails at runtime when listing/filtering files.

## Impact

This affects **all file-based connectors** that inherit from `AbstractFileBasedStreamReader`, including:
- source-sharepoint-enterprise
- source-microsoft-sharepoint
- source-microsoft-onedrive
- source-s3
- source-gcs
- source-azure-blob-storage
- source-sftp-bulk

The issue is triggered when the Terraform provider (or any other integration) normalizes datetime values by stripping microseconds (e.g., `2025-01-01T00:00:00.000000Z` → `2025-01-01T00:00:00Z`).

## Steps to Reproduce

1. Configure any file-based connector with `start_date = "2025-01-01T00:00:00Z"` (no microseconds)
2. Run a sync or discover
3. Observe `ValueError` from `filter_files_by_globs_and_start_date`

## Suggested Fix

Update `filter_files_by_globs_and_start_date` in `file_based_stream_reader.py` to use the flexible `ab_datetime_try_parse` helper from `datetime_helpers` instead of strict `datetime.strptime`. This is consistent with the approach already taken for spec validation in CDK v7.7.1.

## Related

- Oncall issue: `airbytehq/oncall#9390`
- CDK spec fix: `airbytehq/airbyte-python-cdk` PR 887 (CDK v7.7.1)

Requested by @aaronsteers.