Add initial T5Gemma S/S support

[maasir554]

Summary

Adds initial T5Gemma S/S support to gemma.cpp.

This includes:

• T5Gemma S/S model config and tensor metadata
• HF safetensors -> SBS conversion through python/convert_from_safetensors.py
• Encoder-decoder runtime path for fresh seq2seq generation
• BF16 default conversion path for correctness/parity
• Basic docs and focused config/tensor/cache tests

Scope

This PR targets T5Gemma v1 S/S only.

Out of scope:

• T5Gemma 2
• Other T5Gemma sizes
• Multiturn/cache reuse with changing encoder inputs
• Fully optimized T5-specific attention/logits kernels

Local Validation

• bazel test -c opt //:configs_test //:tensor_info_test //:kv_cache_test
• Local optimized smoke test with t5gemma-s-s-it
  • prompt: Hello
  • output begins: I am a new user, ...
  • local speed: roughly 30-35 generated tok/s on MacBook M1, 8GB RAM

Notes

Opened as draft to get early maintainer feedback on scope, naming, and whether this should be split further.

Additional Note: //:gemma_lib_test currently fails locally for me in an existing GemmaAttentionFlash/EMU128 golden mismatch on Apple Silicon; this reproduced outside the T5Gemma changes.

Related to #661

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