[ENG-2914] tidy(salesforce): Support flexible batch response schema

[Cobalt0s]

Description

Salesforce batch response to support a flexible and resilient schema (internal+external).

• Introduces use of codec.RawJSON for batch items to retain both typed (where we use reference id) and raw JSON representations (pass on this data as WriteResult.Data).
• Surfaces the raw provider payload in API responses to prevent data truncation.
• Simplifies error decoding from a strict struct ItemError to map[string]any, as error fields are provider-specific and not consumed internally.

[linear]

ENG-2914 Batch write MVP

[Cobalt0s]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
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• [ENG-2914] tidy(salesforce): Support flexible batch response schema #2277 👈 (View in Graphite)
• feat(codec): Dual marshal preserve RawJSON #2276 : 1 other dependent PR (#2945 )
• feat(testroutines): Subscribe and ReadByIds #3009 : 1 other dependent PR (#3014 )
• feat(subscriptions): Scripts for consuming events #2948
• feat(components): Subscription interfaces #2876
• main

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[Cobalt0s]

Salesforce's BatchWrite implementation changed and uses a new endpoint for create and this solution is no longer applicable.