Fix normalize_string_prefix not lowercasing uppercase T t-string prefix

[JSap0914]

Description

normalize_string_prefix is documented to "make all string prefixes lowercase", and it already normalises F→f and B→b. However, when t-string support was added in #4805 the T→t normalisation was accidentally omitted.

Bug: Black leaves T"..." unchanged instead of normalising it to t"...":

# Before (Black ≥ 26.5 with --target-version py314)
T"hello"   →   T"hello"   # wrong — should be t"hello"
RT"hello"  →   RT"hello"  # wrong — should be Rt"hello"
rT"hello"  →   rT"hello"  # wrong — should be rt"hello"

Fix: Add a single .replace("T", "t") step to the existing chain in normalize_string_prefix.

Verification:

$ pytest tests/test_format.py -k pep_750 -v
PASSED  tests/test_format.py::test_simple_format[pep_750]

AI-assisted: this PR was prepared with the help of an AI coding tool.

Checklist - did you ...

• Implement any code style changes under the --preview style, following the stability policy?
• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

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[JelleZijlstra]

This needs to go into the preview style.

[JSap0914]

T→t prefix normalization is now gated behind Preview.normalize_tstring_prefix, so it only applies in --preview mode as requested.