Cover breakout port generation

[ideaship]

Part of the series tracked in #2562, which explains the ordering and what each PR covers. Based on the preceding PR in the stack, so review only the top commits here.

First scenario overlay: devices that exercise breakout port generation, covering
BREAKOUT_CFG and BREAKOUT_PORTS, which the base fixtures leave empty.

Two devices differ only in how their sub-port speed is established — one derived
from the interface type, one with an explicit NetBox speed — because the
kbps-to-Mbps conversion and the type-derivation fallback are separate code paths
that both feed breakout mode selection.

[sourcery-ai]

Hey - I've found 1 issue

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="tests/e2e/scenario/resources/500-breakout.yml" line_range="12-21" />
<code_context>
+# fields created by 100-base.yml, and take the next free positions in the
+# shared E2E rack so they never collide with the base devices.
+#
+# Both devices use the edgecore-9726-32d-e2e device type (hwsku
+# Accton-AS9726-32D) and break Eth1/1 into a 4x100G group. They differ
+# only in how the sub-port speed reaches NetBox:
+#
+#   e2e-breakout-derived   speed derived from the interface type
+#                          (100gbase-x-qsfp28 -> 100000 Mbps); no explicit
+#                          speed. This is how real deployments model
+#                          breakouts.
+#   e2e-breakout-explicit  the same sub-ports with an explicit NetBox speed
+#                          set in kbps (100000000), the other unit the
+#                          collection step must normalise.
+#
</code_context>
<issue_to_address>
**nitpick:** The scenario comment states that the two devices differ only in how sub-port speed reaches NetBox, but `e2e-breakout-derived` also has a tagged VLAN on `Eth1/5` while `e2e-breakout-explicit` has no corresponding VLAN fixture. The comment is misleading when comparing the resulting coverage and makes the fixture differences harder to understand.

**Suggested fix:** Change the comment to state that the devices differ in speed representation and that only the derived device additionally carries the tagged-VLAN coverage fixture.

```suggestion
# Accton-AS9726-32D) and break Eth1/1 into a 4x100G group. They differ in
# how the sub-port speed is represented in NetBox, and only the derived device
# additionally carries the tagged-VLAN coverage fixture:
#
#   e2e-breakout-derived   speed derived from the interface type
#                          (100gbase-x-qsfp28 -> 100000 Mbps); no explicit
#                          speed. This is how real deployments model
#                          breakouts.
#   e2e-breakout-explicit  the same sub-ports with an explicit NetBox speed
#                          set in kbps (100000000), the other unit the
#                          collection step must normalise.
```
</issue_to_address>

Sourcery assessment

Approved.

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[sourcery-ai]

nitpick: The scenario comment states that the two devices differ only in how sub-port speed reaches NetBox, but e2e-breakout-derived also has a tagged VLAN on Eth1/5 while e2e-breakout-explicit has no corresponding VLAN fixture. The comment is misleading when comparing the resulting coverage and makes the fixture differences harder to understand.

Suggested fix: Change the comment to state that the devices differ in speed representation and that only the derived device additionally carries the tagged-VLAN coverage fixture.

Suggested change

	# Accton-AS9726-32D) and break Eth1/1 into a 4x100G group. They differ
	# only in how the sub-port speed reaches NetBox:
	#
	# e2e-breakout-derived speed derived from the interface type
	# (100gbase-x-qsfp28 -> 100000 Mbps); no explicit
	# speed. This is how real deployments model
	# breakouts.
	# e2e-breakout-explicit the same sub-ports with an explicit NetBox speed
	# set in kbps (100000000), the other unit the
	# collection step must normalise.
	# Accton-AS9726-32D) and break Eth1/1 into a 4x100G group. They differ in
	# how the sub-port speed is represented in NetBox, and only the derived device
	# additionally carries the tagged-VLAN coverage fixture:
	#
	# e2e-breakout-derived speed derived from the interface type
	# (100gbase-x-qsfp28 -> 100000 Mbps); no explicit
	# speed. This is how real deployments model
	# breakouts.
	# e2e-breakout-explicit the same sub-ports with an explicit NetBox speed
	# set in kbps (100000000), the other unit the
	# collection step must normalise.