fix(openai): correctly tag image inputs for chat completions

[Yun-Kim]

Resolves #7737.

This PR adds a step to stringify input messages before tagging in the OpenAI chat completions endpoint. Previously, we had assumed that messages.content would always be a string (which was true until OpenAI recently added the image input feature to the chat completions endpoint), but it can now be an array of str-str dictionaries.

Testing Strategy

Regression tests have been added, and manual testing has also confirmed that the error reported on #7737 does not appear.

Checklist

• Change(s) are motivated and described in the PR description.
• Testing strategy is described if automated tests are not included in the PR.
• Risk is outlined (performance impact, potential for breakage, maintainability, etc).
• Change is maintainable (easy to change, telemetry, documentation).
• Library release note guidelines are followed. If no release note is required, add label changelog/no-changelog.
• Documentation is included (in-code, generated user docs, public corp docs).
• Backport labels are set (if applicable)

Reviewer Checklist

• Title is accurate.
• No unnecessary changes are introduced.
• Description motivates each change.
• Avoids breaking API changes unless absolutely necessary.
• Testing strategy adequately addresses listed risk(s).
• Change is maintainable (easy to change, telemetry, documentation).
• Release note makes sense to a user of the library.
• Reviewer has explicitly acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment.
• Backport labels are set in a manner that is consistent with the release branch maintenance policy
• If this PR touches code that signs or publishes builds or packages, or handles credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-12-01 17:19:04

Comparing candidate commit 0e3c26b in PR branch yunkim/fix-openai-chat-image-input with baseline commit 215f2bf in branch main.

Found 1 performance improvements and 0 performance regressions! Performance is the same for 89 metrics, 0 unstable metrics.

scenario:flasksimple-appsec-telemetry

• 🟩 max_rss_usage [-1.139MB; -1.020MB] or [-3.164%; -2.836%]

[github-actions]

The backport to 2.2 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-2.2 2.2
# Navigate to the new working tree
cd .worktrees/backport-2.2
# Create a new branch
git switch --create backport-7759-to-2.2
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 aeb1e47f27f9bfe476c825287aa8465382dec4cb
# Push it to GitHub
git push --set-upstream origin backport-7759-to-2.2
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.2

Then, create a pull request where the base branch is 2.2 and the compare/head branch is backport-7759-to-2.2.

[github-actions]

The backport to 2.3 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-2.3 2.3
# Navigate to the new working tree
cd .worktrees/backport-2.3
# Create a new branch
git switch --create backport-7759-to-2.3
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 aeb1e47f27f9bfe476c825287aa8465382dec4cb
# Push it to GitHub
git push --set-upstream origin backport-7759-to-2.3
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.3

Then, create a pull request where the base branch is 2.3 and the compare/head branch is backport-7759-to-2.3.