[MagpieTTS][TTS] ja longform fix and StrEnum import fix

[subhankar-ghosh]

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What does this PR do ?

The _initialize_chunked_attn_prior function writes attention prior weights to 5 consecutive text token positions starting at
current_starting_point. The offsets 0–4 are hardcoded with no bounds check. If the chunk has fewer than current_starting_point + 5 tokens, index + 4 goes out of bounds.
This surfaces for Japanese longform because:

1. Longform splits text into sentence chunks
2. Japanese sentences can be very short (e.g., 「日本の棚田百選」に選出されている。)
3. The Japanese phoneme tokenizer produces very few tokens for short sentences — sometimes just 4 including EOS
4. max_text_len = 4 → valid indices are 0–3 → writing to index 4 crashes

StrEnum was added to Python's standard library enum module in Python 3.11. On Python 3.10, from enum import StrEnum raises an
ImportError at import time, which means the entire eou_classifier module fails to load — crashing any environment running
Python 3.10.

Collection: [Note which collection this PR will affect]

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[github-actions]

[🤖]: Hi @subhankar-ghosh 👋,

We wanted to let you know that a CICD pipeline for this PR just finished successfully.

So it might be time to merge this PR or get some approvals.

[rfejgin]

Looks good but please see comment

[rfejgin]

I'd suggest to name at least one of _idx and idx differently to help distinguish their meaning more easily

[rfejgin]

@subhankar-ghosh fyi

[blisc]

Done in #15551

[github-actions]

[🤖]: Hi @subhankar-ghosh 👋,

We wanted to let you know that a CICD pipeline for this PR just finished successfully.

So it might be time to merge this PR or get some approvals.