feat(block-kit): add block element examples

[zimeg]

Summary

Adds dedicated block element examples under block-kit/, one file per element in src/elements/ with a matching test in tests/elements/.

Each example mirrors the JSON payload(s) from the element's docs.slack.dev reference page verbatim — hosted in a valid block (or input) where the element requires one, so each is a runnable payload rather than a bare element that can't be sent on its own. Where a reference page documents multiple payloads, every one is exemplified, and each test asserts the complete reference JSON.

The example directory is elements/ (not block_elements/) so the layout reads blocks/ + elements/, matching the docs.slack.dev path structure where Blocks and Block elements are siblings under Block Kit — and matching the sibling bolt-js-examples and bolt-java-examples PRs. The SDK's own slack_sdk.models.blocks.block_elements imports are unaffected by the rename.

This PR is the home for block elements the SDK implements; elements not yet implemented in the SDK are a separate follow-up.

Elements covered

34 elements, 49 examples.

Element	Examples
Broadcast	1
Button	4 (regular, primary, link, hosted section + actions)
Channel	1
Checkboxes	1
Color	1
Date	1
Date picker	1
Datetime picker	1
Email input	1
Emoji	1
Feedback buttons	1
File input	1
Icon button	1
Image	3 (image_url, slack_file by url, slack_file by id)
Link	1
Multi-select menu	5 (static, external, users, conversations, channels)
Number input	1
Overflow menu	1
Plain-text input	1
Radio button group	1
Rich text input	1
Rich text list	2
Rich text preformatted	1
Rich text quote	1
Rich text section	1
Select menu	5 (static, external, users, conversations, channels)
Team	1
Text	1
Time picker	1
URL input	1
URL source	2
User	1
Usergroup	1
Workflow button	1

Deferred

The button reference page also documents an agent_prompt variant (Slackbot hand-off), which the SDK does not yet implement — deferred here rather than shipping an example whose serialized payload wouldn't include the field.

Testing

ruff check, ruff format --check, mypy, and pytest all pass in block-kit/ (83 tests).

[zimeg]

🐍 Notes on more review after comparing to adjacent slack-samples/bolt-js-examples#215!

[zimeg]

📝 note: This pattern exists just for these returns but matches current implementation of this example:

bolt-python-examples/block-kit/src/blocks/rich_text.py

Lines 11 to 69 in ab9308d

	def example01() -> list[RichTextBlock]:
	"""
	Displays formatted, structured representation of text.
	https://docs.slack.dev/reference/block-kit/blocks/rich-text-block/
	Four basic rich text section examples (basic, bold, italic, strikethrough).
	"""
	blocks = [
	RichTextBlock(
	elements=[
	RichTextSectionElement(
	elements=[
	RichTextElementParts.Text(
	text="Hello there, I am a basic rich text block!"
	)
	]
	)
	]
	),
	RichTextBlock(
	elements=[
	RichTextSectionElement(
	elements=[
	RichTextElementParts.Text(text="Hello there, "),
	RichTextElementParts.Text(
	text="I am a bold rich text block!", style={"bold": True}
	),
	]
	)
	]
	),
	RichTextBlock(
	elements=[
	RichTextSectionElement(
	elements=[
	RichTextElementParts.Text(text="Hello there, "),
	RichTextElementParts.Text(
	text="I am an italic rich text block!",
	style={"italic": True},
	),
	]
	)
	]
	),
	RichTextBlock(
	elements=[
	RichTextSectionElement(
	elements=[
	RichTextElementParts.Text(text="Hello there, "),
	RichTextElementParts.Text(
	text="I am a strikethrough rich text block!",
	style={"strike": True},
	),
	]
	)
	]
	),
	]
	return blocks

[zimeg]

🌲 note: These parameters within the workflow_button trigger don't have classes available so for now the example uses plain objects.

[zimeg]

☕ note: Example implementation shows a structure here has precedence!

🔗 https://github.com/slack-samples/bolt-java-examples/blob/e7cba09313175e99fe1c4438ae8f32609d56e7d7/block-kit/src/main/java/elements/WorkflowButton.java#L28-L37

[zimeg]

⌛ note: This is perhaps unexpected working upstream according to some metadata but we're matching what's shown in HTML at this time.

[zimeg]

🚢 Merging after match with slack-samples/bolt-java-examples#69 and slack-samples/bolt-js-examples#215!