feat: add HAYSTACK_PROGRESS_BARS environment variable support

[luojiyin1987]

Summary

Fix #8782 - Add support for HAYSTACK_PROGRESS_BARS environment variable to globally control progress bar behavior across all Haystack components.

Problem

Haystack 1.x had a HAYSTACK_PROGRESS_BARS environment variable that allowed users to globally disable progress bars. This functionality was not migrated to Haystack 2.x, and setting standard tqdm environment variables like TQDM_DISABLE doesn't work because components explicitly pass disable=not self.progress_bar to tqdm.

Solution

Created a centralized get_progress_bar_setting() utility function that:

1. Checks the HAYSTACK_PROGRESS_BARS environment variable first
2. Falls back to the component's progress_bar parameter if env var is not set
3. Supports both enable/disable values (0/1, true/false, on/off, etc.)

This allows users to:

• Disable all progress bars globally: export HAYSTACK_PROGRESS_BARS=0
• Enable all progress bars globally: export HAYSTACK_PROGRESS_BARS=1
• Use component defaults when env var is not set

Serialization Fix

The implementation saves the original progress_bar parameter value as _progress_bar_param for serialization, while self.progress_bar contains the env-resolved value for runtime. This prevents the environment variable effect from being "baked into" serialized pipeline configs.

Changes

• New file: haystack/utils/progress_helper.py - Core utility function
• Modified: haystack/utils/__init__.py - Export the new function
• New file: test/utils/test_progress_helper.py - Comprehensive tests
• Modified: All embedder and evaluator components with progress_bar parameter:
  • OpenAIDocumentEmbedder
  • HuggingFaceAPIDocumentEmbedder
  • AzureOpenAIDocumentEmbedder
  • SentenceTransformersDocumentEmbedder
  • SentenceTransformersTextEmbedder
  • SentenceTransformersSparseDocumentEmbedder
  • SentenceTransformersDocImageEmbedder
  • LLMEvaluator
  • FaithfulnessEvaluator
  • ContextRelevanceEvaluator
  • MarkdownToDocument converter

Test Plan

• Unit tests for get_progress_bar_setting() with various env values
• Manual verification: HAYSTACK_PROGRESS_BARS=0 disables progress bars
• Manual verification: HAYSTACK_PROGRESS_BARS=1 enables progress bars
• Verified serialization preserves original parameter value (not env-resolved)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally

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