chore(llmobs): make mlapp error message more helpful [backport 3.9]

[github-actions]

Backport 6931943 from #13714 to 3.9.

Makes the (lack of) ML App error message more detailed, specifically to state the env var / config that needs to be set to avoid the error in the future.

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:

ddtrace/llmobs/_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: 272 ± 2 ms.

The average import time from base is: 274 ± 2 ms.

The import time difference between this PR and base is: -2.03 ± 0.08 ms.

Import time breakdown

The following import paths have shrunk:

ddtrace.auto 1.977 ms (0.73%)

ddtrace.bootstrap.sitecustomize 1.307 ms (0.48%)

ddtrace.bootstrap.preload 1.307 ms (0.48%)

ddtrace.internal.remoteconfig.client 0.652 ms (0.24%)

ddtrace 0.670 ms (0.25%)

ddtrace.internal._unpatched 0.030 ms (0.01%)

json 0.030 ms (0.01%)

json.decoder 0.030 ms (0.01%)

re 0.030 ms (0.01%)

enum 0.030 ms (0.01%)

types 0.030 ms (0.01%)

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-06-23 15:55:24

Comparing candidate commit 1d6ddd2 in PR branch backport-13714-to-3.9 with baseline commit 03c93b7 in branch 3.9.

Found 0 performance improvements and 6 performance regressions! Performance is the same for 547 metrics, 3 unstable metrics.

scenario:iastaspects-format_map_aspect

• 🟥 execution_time [+466.534ns; +523.034ns] or [+14.610%; +16.380%]

scenario:iastaspects-lstrip_aspect

• 🟥 execution_time [+1.377µs; +1.559µs] or [+10.615%; +12.024%]

scenario:iastaspects-strip_aspect

• 🟥 execution_time [+1.400µs; +1.576µs] or [+10.803%; +12.157%]

scenario:iastaspects-translate_aspect

• 🟥 execution_time [+235.003ns; +275.108ns] or [+7.057%; +8.261%]

scenario:iastaspectsospath-ospathnormcase_aspect

• 🟥 execution_time [+413.802ns; +461.026ns] or [+11.976%; +13.343%]

scenario:iastaspectsospath-ospathsplitext_aspect

• 🟥 execution_time [+601.363ns; +672.261ns] or [+13.253%; +14.816%]

[Yun-Kim]

Closing as this isn't a fix and doesn't need to be backported.