fix(security): enforce datasource access control in get_samples()

[rusackas]

SUMMARY

This PR fixes a security vulnerability (issue #31944) where users with "can samples on Datasource" permission could read data samples from datasets they don't have proper access to.

Root Cause:
The get_samples() function in superset/views/datasource/utils.py was creating QueryContext instances and calling get_payload() directly without first enforcing access control through raise_for_access().

This allowed users who only had the "can samples on Datasource" permission to bypass datasource-level security checks and read samples from datasets they shouldn't have access to.

Fix:
Added raise_for_access() calls on both the samples and count_star query contexts before fetching any data. This ensures users must have proper datasource access (schema access, datasource access, or ownership) before samples can be retrieved.

Code Change:

try:
    # Enforce access control before fetching data.
    # This prevents users with "can samples on Datasource" permission from
    # reading samples from datasets they don't have access to.
    samples_instance.raise_for_access()
    count_star_instance.raise_for_access()

    count_star_data = count_star_instance.get_payload()["queries"][0]
    # ... rest of the function

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A - This is a security fix with no UI changes.

TESTING INSTRUCTIONS

1. Create a user with only "can samples on Datasource" permission
2. Create a dataset that the user does NOT have access to (no datasource_access, schema_access, or ownership)
3. Before this fix: User could access /datasource/samples?datasource_id=<id>&datasource_type=table and retrieve samples
4. After this fix: User receives a 403/401 error when attempting to access samples from datasets they don't have permission to access

Unit Tests:
The PR includes comprehensive unit tests that verify:

• raise_for_access() is called before fetching data
• Security exceptions are properly raised when access is denied
• Both samples and count_star query contexts are checked for access

ADDITIONAL INFORMATION

• Has associated issue: Fixes get_samples() doesn't raise for access #31944
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

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