rust(feat): expose measurement received time in MCP get_data

[rowen-sift]

Supersedes #817, which closed when its branch was renamed.

Description

MCP get_data now accepts include_received_at (alias includeReceivedAt). When it is true, the tool sends that flag on GetDataRequest and, when the service returns sift_received_at in extras, writes a receipt-time column next to the channel.

• timestamp_unix_nanos stays the measurement generation time.
• The new column is Int64 unix nanos, named <channel> {channel_id="...", received_at="true", ...}. Cells are null when that sample has no receipt time, or when the channel has no sample on the row.
• The column is omitted when the service returns no receipt times. Enum and bit-field channels have no receipt time.
• Leaving the flag off keeps the previous request and schema.
• upload_dataset skips received_at="true" columns so they are not ingested as channels.

Verification

• cargo test -p sift_mcp --lib: 577 passed.
• Service tests cover: the flag is omitted unless asked; receipt times land in the column, including a null receipt; a missing generation timestamp does not shift the next receipt time; merged rows null the receipt column where that channel has no sample; pagination appends receipt times and backfills earlier pages that had none.
• get_data parameter deserialization accepts both include_received_at and includeReceivedAt, and defaults to false.

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