[3.15] appsec_integrations_fastapi — enable on Python 3.15

[vlad-scherbich]

Tracked under parent: #17809

Tier: B
Section: ASM — @DataDog/asm-python

---

Original row from the parent tracker:

• appsec_integrations_fastapi — only the old fastapi sub-venvs (==0.86.0, ==0.94.1) are capped at "3.13" (riotfile.py:340-345); the newer sub-venvs (~=0.114.2, latest) already test 3.14+ once IAST works there. Real blocker is likely IAST runtime, not the fastapi integration itself — i.e. this row is implicitly gated on the IAST 3.15 row above.

---

To claim this row, self-assign on this issue (top-right "Assignees" → "assign yourself") or leave a Taking this comment.

To close this row, open a PR using the python_315_bump template (added in #17791; open via ?template=python_315_bump.md) and include Closes #<this-issue-number> in the description. That auto-closes this issue on merge, which auto-flips the corresponding checkbox on the parent tracker (#17809).