feat: ChatPromptBuilder copies entire ChatMessage rather than copying content field only

[vblagoje]

Why:

The main motivation for this PR is the need to ensure that all ChatMessage fields including meta dict keys/values are copied properly. In the ChatPromptBuilder when we render templates, currently, we just create copy of the content and create a new ChatMessage using that content instead of proper copy of all fields (including meta which might carry important contextual info). Because we don't do proper copies of the ChatMessage in this case we can't do things like prompt caching (flags in meta dict are currently lost) and perhaps some other important payloads users might store in meta.

See this issue for additional context.

• fixes Copy ChatMessage meta fields in ChatPromptBuilder haystack-core-integrations#1011

What:

• Modified the chat prompt builder logic to utilize deep copy for message processing, replacing the previous method of selectively copying attributes and creating new message instances based on the content and origin (user/system).

How can it be used:

No visible changes to user, all modifications are internal to ChatPromptBuilder

How did you test it:

• Uses exhaustive existing tests
• Adds a new test_run_with_meta in test/components/builders/test_chat_prompt_builder.py

[coveralls]

Pull Request Test Coverage Report for Build 10668756300

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.002%) to 90.198%

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Files with Coverage Reduction	New Missed Lines	%
components/builders/chat_prompt_builder.py	1	98.51%

Totals
Change from base Build 10664748045:	0.002%
Covered Lines:	7003
Relevant Lines:	7764

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💛 - Coveralls

[silvanocerza]

Looks good 👍