fix(retrieval): repair Ministral3 training recipe (#3441)

* fix(retrieval): use registered Ministral tokenizer

Signed-off-by: Yuhe Zhang <yuhez@nvidia.com>

* test(retrieval): add Ministral nightly smoke

Signed-off-by: Yuhe Zhang <yuhez@nvidia.com>

* fix(retrieval): dequantize native fp8 backbones

Signed-off-by: Yuhe Zhang <yuhez@nvidia.com>

* fix(retrieval): support strict tokenizer backends

Signed-off-by: Yuhe Zhang <yuhez@nvidia.com>

* fix(retrieval): use BF16 Ministral checkpoint

Signed-off-by: Yuhe Zhang <yuhez@nvidia.com>

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Signed-off-by: Yuhe Zhang <yuhez@nvidia.com>