fix(iast): invalid f-string type conversions exception in format_value_aspect

[avara1986]

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: 237 ± 4 ms.

The average import time from base is: 240 ± 5 ms.

The import time difference between this PR and base is: -3.0 ± 0.2 ms.

Import time breakdown

The following import paths have shrunk:

ddtrace.auto 2.025 ms (0.86%)

ddtrace.bootstrap.sitecustomize 1.358 ms (0.57%)

ddtrace.bootstrap.preload 1.358 ms (0.57%)

ddtrace.internal.products 1.358 ms (0.57%)

ddtrace.internal.remoteconfig.client 0.652 ms (0.28%)

ddtrace 0.667 ms (0.28%)

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-04-10 18:12:46

Comparing candidate commit fef883b in PR branch avara1986/fstring_exception with baseline commit 9d2d4e6 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 496 metrics, 2 unstable metrics.