feat(minimax): add per_call_cost for video and music models (#79)

Video and music modality models are billed per generation rather than
per token. Without this field the librefang runtime metering layer
records every call as $0 and emits a warning. Sourced from
https://platform.minimax.io/docs/guides/pricing-paygo.

- Hailuo 2.3 Fast:  $0.33 per call (1080P/6s upper bound)
- Hailuo 2.3:       $0.56 per call (1080P/6s upper bound)
- Hailuo 02:        $0.56 per call (1080P/6s upper bound)
- Music 2.6:        $0.15 per up-to-5-minute track
- Lyrics gen:       $0.01 per song

Also adds per_call_cost to schema.toml so the field is documented
alongside the other cost fields.

providers/minimax.toml

@@ -155,6 +155,7 @@ tier = "fast"
 modality = "video"
 input_cost_per_m = 0.0
 output_cost_per_m = 0.0
+per_call_cost = 0.33
 supports_tools = false
 supports_vision = false
 supports_streaming = false
@@ -168,6 +169,7 @@ tier = "frontier"
 modality = "video"
 input_cost_per_m = 0.0
 output_cost_per_m = 0.0
+per_call_cost = 0.56
 supports_tools = false
 supports_vision = false
 supports_streaming = false
@@ -181,6 +183,7 @@ tier = "smart"
 modality = "video"
 input_cost_per_m = 0.0
 output_cost_per_m = 0.0
+per_call_cost = 0.56
 supports_tools = false
 supports_vision = false
 supports_streaming = false
@@ -196,6 +199,7 @@ tier = "frontier"
 modality = "music"
 input_cost_per_m = 0.0
 output_cost_per_m = 0.0
+per_call_cost = 0.15
 supports_tools = false
 supports_vision = false
 supports_streaming = false
@@ -209,6 +213,7 @@ tier = "fast"
 modality = "music"
 input_cost_per_m = 0.0
 output_cost_per_m = 0.0
+per_call_cost = 0.01
 supports_tools = false
 supports_vision = false
 supports_streaming = false