fix(iast): invalid f-string type conversions exception in format_value_aspect [backport 3.4]

[github-actions]

Backport f807b8e from #13156 to 3.4.

This fixes a bug with f-strings when IAST is enabled: when building an invalid string that should raise an "Unknown format code X" exception, the error wasn't being triggered in certain cases. like

 f"{text:05d}"

Should raise a ValueError("Unknown format code 'd' for object of type 'str'")

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:

releasenotes/notes/iast-fstring-exception-d460a37b6e44f972.yaml         @DataDog/apm-python
ddtrace/appsec/_iast/_taint_tracking/aspects.py                         @DataDog/asm-python
tests/appsec/iast/aspects/test_str_py3.py                               @DataDog/asm-python

[github-actions]

Bootstrap import analysis

Comparison of import times between this PR and base.

Summary

The average import time from this PR is: 235 ± 4 ms.

The average import time from base is: 235 ± 3 ms.

The import time difference between this PR and base is: 0.4 ± 0.2 ms.

Import time breakdown

The following import paths have shrunk:

ddtrace.auto 1.731 ms (0.74%)

ddtrace.bootstrap.sitecustomize 1.135 ms (0.48%)

ddtrace.bootstrap.preload 1.135 ms (0.48%)

ddtrace.internal.products 1.135 ms (0.48%)

ddtrace.internal.remoteconfig.client 0.573 ms (0.24%)

ddtrace 0.596 ms (0.25%)

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-04-11 14:22:58

Comparing candidate commit 989920d in PR branch backport-13156-to-3.4 with baseline commit 306489d in branch main.

Found 0 performance improvements and 7 performance regressions! Performance is the same for 491 metrics, 2 unstable metrics.

scenario:iast_aspects-ospathbasename_aspect

• 🟥 execution_time [+1.279µs; +1.312µs] or [+39.095%; +40.112%]

scenario:iast_aspects-ospathdirname_aspect

• 🟥 execution_time [+1.498µs; +1.555µs] or [+40.355%; +41.889%]

scenario:iast_aspects-ospathjoin_aspect

• 🟥 execution_time [+1.605µs; +1.649µs] or [+31.645%; +32.498%]

scenario:iast_aspects-ospathnormcase_aspect

• 🟥 execution_time [+1.209µs; +1.243µs] or [+47.752%; +49.074%]

scenario:iast_aspects-ospathsplit_aspect

• 🟥 execution_time [+1.415µs; +1.451µs] or [+36.339%; +37.252%]

scenario:iast_aspects-ospathsplitdrive_aspect

• 🟥 execution_time [+1.267µs; +1.299µs] or [+46.073%; +47.219%]

scenario:iast_aspects-ospathsplitext_aspect

• 🟥 execution_time [+1.260µs; +1.292µs] or [+34.103%; +34.967%]