feat(model): add TaskCardBlock and UrlSourceElement

[srtaalej]

Summary

• Adds TaskCardBlock layout block (task_card type) with all fields from docs
• Adds UrlSourceElement for the sources field
• Registers in GsonLayoutBlockFactory, adds Blocks.taskCard() helper
• Includes parse and build tests in BlockKitTest and BlocksTest

Fields

Field	Type	Required
type	"task_card"	yes
task_id	String	yes
title	String	yes
status	String (in_progress, complete, error)	yes
details	RichTextBlock	no
output	RichTextBlock	no
sources	List<UrlSourceElement>	no
block_id	String	no
icon	SlackIconObject	no
hide_title	Boolean	no

Note: The docs example uses "pending" as a status value, but the API rejects it with invalid_blocks. Only in_progress, complete, and error are accepted.

Example usage

import static com.slack.api.model.block.Blocks.*;

var blocks = asBlocks(
    taskCard(t -> t
        .taskId("task_1")
        .title("Fetching weather data")
        .status("complete")
        .output(RichTextBlock.builder()
            .elements(Arrays.asList(
                RichTextSectionElement.builder()
                    .elements(Arrays.asList(
                        RichTextSectionElement.Text.builder()
                            .text("Found weather data for Chicago from 2 sources")
                            .build()
                    ))
                    .build()
            ))
            .build())
        .sources(Arrays.asList(
            UrlSourceElement.builder().url("https://weather.com/").text("weather.com").build(),
            UrlSourceElement.builder().url("https://www.accuweather.com/").text("accuweather.com").build()
        ))
    ),
    taskCard(t -> t
        .taskId("task_2")
        .title("Analyzing results")
        .status("in_progress")
    ),
    taskCard(t -> t
        .taskId("task_3")
        .title("Send notification")
        .status("error")
        .hideTitle(true)
        .output(RichTextBlock.builder()
            .elements(Arrays.asList(
                RichTextSectionElement.builder()
                    .elements(Arrays.asList(
                        RichTextSectionElement.Text.builder()
                            .text("Failed to reach notification service")
                            .build()
                    ))
                    .build()
            ))
            .build())
    )
);

Test plan

• parseTaskCardBlock — deserializes JSON from docs example
• buildTaskCardBlock — constructs via Blocks.taskCard() helper
• testTaskCard — static helper test in BlocksTest
• Tested in Slack UI via Socket Mode app with chatPostMessage

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.84%. Comparing base (e30ba9d) to head (5023106).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1638   +/-   ##
=========================================
  Coverage     72.84%   72.84%           
- Complexity     4533     4535    +2     
=========================================
  Files           482      482           
  Lines         14423    14425    +2     
  Branches       1512     1512           
=========================================
+ Hits          10506    10508    +2     
  Misses         3022     3022           
  Partials        895      895           

Flag	Coverage Δ
jdk-14	72.84% <100.00%> (+<0.01%)	⬆️

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