Fix CodeRabbit issues and add test infrastructure

  Addresses all CodeRabbit critical issues:

  1. Case-insensitive error detection (CRITICAL)
     - Use strings.ToLower() for error matching in retry logic
     - Now catches '404 Not Found', 'No file found', 'FILE NOT FOUND', etc.
     - Prevents immediate failure on HTTP 404 responses

  2. Guard against invalid maxRetries configuration (CRITICAL)
     - Added validation to fail fast when maxRetries <= 0
     - Returns clear error message about configuration issue

  3. Comprehensive edge case tests (MAJOR)
     - ZeroMaxRetries: validates error on maxRetries=0
     - NegativeMaxRetries: validates error on maxRetries=-1
     - ErrorMessageCaseInsensitivity: tests all case variations
     - Covers '404 Not Found', 'Unauthorized', 'Rate limit' scenarios

  4. Test infrastructure improvements
     - Created testdata files (sample.txt, sample1-3.txt) for acceptance tests
     - Added testAccCheckOpenAIVectorStoreFileDestroy verification
     - Made OPENAI_ORGANIZATION_ID optional (supports personal accounts)
     - Added acceptance-tests.yml workflow for secure CI/CD testing
     - Added cost warning in workflow output

  All tests pass (33.354s) with proper timing validation.

Author: pabloinigo

testdata/sample.txt

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+This is a sample text file for testing OpenAI vector store file creation.
+
+It contains basic information about Terraform testing with the OpenAI provider.
+Testing the retry logic for eventual consistency issues when creating vector store files.
+
+Key points:
+- Vector stores are used for assistants and file search
+- Files need to be uploaded first before adding to vector store
+- The OpenAI API may experience eventual consistency delays
+- This test validates the retry logic handles these delays correctly
+
+End of test file.

testdata/sample1.txt

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+Sample file 1 for testing multiple vector store file creation.
+
+This file is part of a test that validates the retry logic when creating
+multiple vector store files simultaneously, which can trigger eventual
+consistency issues with the OpenAI API.
+
+Test scenario: Create 3 files concurrently and verify all succeed with retry logic.

testdata/sample2.txt

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+Sample file 2 for testing multiple vector store file creation.
+
+This is the second file in a batch of concurrent file uploads to test
+the retry logic when the OpenAI API experiences eventual consistency issues.
+
+The test ensures that even when multiple files are created at the same time,
+the provider handles any temporary "file not found" errors gracefully.

testdata/sample3.txt

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+Sample file 3 for testing multiple vector store file creation.
+
+This is the third and final file in the concurrent upload test batch.
+It validates that the exponential backoff retry logic works correctly
+when multiple API calls are made simultaneously.
+
+Expected behavior:
+- All three files should be successfully added to the vector store
+- Any temporary "file not found" errors should be retried automatically
+- Maximum of 5 retry attempts with exponential backoff (1s, 2s, 4s, 8s, 16s)