Add ContentType support for Blob and Chunk resources

[Copilot]

Adds fields to track and forward Content-Type headers from source to destination during blob transfers.

Changes

• BlobStatus.ContentType: Captures Content-Type from source HEAD response
• ChunkSpec.SourceResponseHeadersToDestination: Whitelist of source headers to forward to destinations
• blob_from_chunk_controller: Extracts Content-Type during HEAD chunk processing
• blob_to_chunk_controller: Sets header forwarding list when ContentType is known
• chunk_runner: Implements header forwarding from source response to destination requests

Example

// BlobStatus now includes content type from source
type BlobStatus struct {
    Total        int64  `json:"total,omitempty"`
    ContentType  string `json:"contentType,omitempty"`  // New field
    // ...
}

// ChunkSpec specifies which headers to forward
type ChunkSpec struct {
    Source      ChunkHTTP   `json:"source"`
    Destination []ChunkHTTP `json:"destination,omitempty"`
    SourceResponseHeadersToDestination []string `json:"sourceResponseHeadersToDestination,omitempty"`  // New field
    // ...
}

When a blob has a known ContentType, chunks are created with SourceResponseHeadersToDestination: ["Content-Type"], causing the runner to forward the Content-Type header from source to all destination PUT requests.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Support ContentType for blob and chunk</issue_title>
<issue_description>- Add a field to the Chunk to describe which Source's response headers are passed to the Destination

• Add a field to the Blob that respects the ContentType of the source.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Support ContentType for blob and chunk #66

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