fix(anthropic): handle malformed streamed responses gracefully

[sabrenner]

Checklist

• PR author has checked that all the criteria below are met
• The PR description includes an overview of the change
• The PR description articulates the motivation for the change
• The change includes tests OR the PR description describes a testing strategy
• The PR description notes risks associated with the change, if any
• Newly-added code is easy to change
• The change follows the library release note guidelines
• The change includes or references documentation updates if necessary
• Backport labels are set (if applicable)

Reviewer Checklist

• Reviewer has checked that all the criteria below are met
• Title is accurate
• All changes are related to the pull request's stated goal
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Newly-added code is easy to change
• Release note makes sense to a user of the library
• If necessary, author has 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

[github-actions]

CODEOWNERS have been resolved as:

tests/contrib/anthropic/cassettes/anthropic_completion_stream_malformed.yaml  @DataDog/ml-observability
ddtrace/contrib/internal/anthropic/_streaming.py                        @DataDog/ml-observability
tests/contrib/anthropic/test_anthropic.py                               @DataDog/ml-observability
tests/contrib/anthropic/test_anthropic_llmobs.py                        @DataDog/ml-observability

[github-actions]

Bootstrap import analysis

Comparison of import times between this PR and base.

Summary

The average import time from this PR is: 276 ± 6 ms.

The average import time from base is: 277 ± 2 ms.

The import time difference between this PR and base is: -1.3 ± 0.2 ms.

Import time breakdown

The following import paths have shrunk:

ddtrace.auto 1.894 ms (0.69%)

ddtrace.bootstrap.sitecustomize 1.222 ms (0.44%)

ddtrace.bootstrap.preload 1.222 ms (0.44%)

ddtrace.internal.remoteconfig.client 0.631 ms (0.23%)

ddtrace 0.673 ms (0.24%)

ddtrace.internal._unpatched 0.023 ms (0.01%)

[github-actions]

This pull request has been automatically closed after a period of inactivity.
After this much time, it will likely be easier to open a new pull request with the
same changes than to update this one from the base branch. Please comment or reopen
if you think this pull request was closed in error.