add implementation for UpdateItem

[csongor-halmai-oolio]

This PR implements the UpdateItem() functionality for DynamoDB. It has a decent amount of unit tests as well.

Summary by CodeRabbit

• New Features
  • Added UpdateItem support to the write API, allowing updates via optional update expressions and expression attribute values.
  • Introduced UpdateOption helpers to configure DynamoDB return values, condition expression behavior, and capacity/metrics reporting.
• Documentation
  • Added an UpdateItem README section with an example, including an increment operation and ALL_NEW return values.
• Tests
  • Added TestUpdateItem covering insert/update, increment behavior, and condition-failure handling.

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Latest commit: 602e6d7

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📥 Commits

Reviewing files that changed from the base of the PR and between f5289bd and 602e6d7.

📒 Files selected for processing (1)

• interface.go

🚧 Files skipped from review as they are similar to previous changes (1)

• interface.go

---

📝 Walkthrough

Walkthrough

Added UpdateItem to the write API, implemented request construction with option helpers, documented usage in the README, and added table-driven tests for update, increment, and conditional-error cases.

Changes

UpdateItem API and coverage

Layer / File(s)	Summary
UpdateItem API surface and options interface.go, update_item.go, README.md	WriteAPI adds UpdateItem, UpdateOption helpers set UpdateItemInput fields, and the README shows an UpdateItem call with WithReturnValues("ALL_NEW").
UpdateItem request flow update_item.go	Client.UpdateItem builds dynamodb.UpdateItemInput from keys and update expression, applies options, calls DynamoDB, logs errors, and returns output.
UpdateItem table-driven coverage tests/updateitem_test.go	TestUpdateItem runs table-driven insert, update, increment, and conditional-failure cases, then checks returned attributes or expected error substrings.

🎯 3 (Moderate) | ⏱️ ~20 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely summarizes the main change: adding an UpdateItem implementation.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

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• Create stacked PR
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---

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Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@interface.go`:
- Line 31: The UpdateItem method signature in interface.go references
dynamodb.UpdateItemOutput, but the file only imports the DynamoDB types package,
so it will not compile. Update the imports in interface.go to include the
dynamodb service package alongside types, and keep the UpdateItem return type
and any related references consistent with that import.

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---

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Configuration used: Organization UI

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📥 Commits

Reviewing files that changed from the base of the PR and between 3016a5d and f5289bd.

📒 Files selected for processing (2)

• README.md
• interface.go

✅ Files skipped from review due to trivial changes (1)

• README.md