chore(test): upgrade the mock collector and drop the seed workaround (#410)

The pinned mock-collector image dated from October 2022, 13 commits behind
the tool's master. Move it to c6b91a0e, which carries the fix for the
first-insert race in SegmentItems/LogItems
(apache/skywalking-agent-test-tool#65): when two processes reported their
first segment for the same service name concurrently, the collector
silently dropped one while both reporters got HTTP 200.

sw_fork_support worked around that race by seeding the service name with a
parent-only /ping request and waiting for the collector to register it
before triggering the concurrent parent/child reports. The collector no
longer needs the help, so the endpoint, the seed step and the extra
expected segment are gone and the test is back to asserting exactly the
cross-fork trace it is about.

The upgrade also picks up the validator changes made since 2022, one of
which affected us. LogAssert now sorts both the expected and the actual
logs by their body text before comparing them pairwise
(apache/skywalking-agent-test-tool#59), and the sort uses the raw expected
string, so a matcher such as `text: not null` participates as the literal
"not null". sw_loguru matches its two logging-module records that way
while the records themselves led with the default layout's timestamp, so
they sorted first and their placeholders last, inverting the pairing.
Pin a layout that leads with the logger name, which sorts after the
placeholders and orders the two records deterministically, and reorder the
expected entries to match; SWFormatter is still exercised. Its
expected.data.yml is the only one in the tree with a non-empty logItems
block, so no other test is affected.

Also refresh CLAUDE.md: supported Python and grpcio floor, the current
plugin list, the agent's fork/prefork lifecycle, and the plugin-test
validation notes.