[3.15] IAST 3.15

[vlad-scherbich]

Tracked under parent: #17809

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

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Original row from the parent tracker:

• IAST 3.15 — initial commit 84ad6c9f2b feat(iast): enable IAST on Python 3.15 — taint tracking and AST patching verified working lives on vlad/ddtracepy-315-tracing-wrapping for reference, but no PR, no owner. dd-trace-py v3.16.0 wheels shipped without IAST on 3.14; the same fallback (skip on 3.15) is acceptable here. Needs @DataDog/asm-python to validate, take ownership, and ship.

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