controlplane/telemetry: keep newest samples on partial buffer overcapacity

[ayushsingh82]

Summary

Fixes #4149.

• Tick()'s failed-retry path (controlplane/telemetry/internal/telemetry/submitter.go) replaced the binary "does the whole unwritten remainder fit, yes/no" check with room := capacity - bufLen.
• When the unwritten backlog doesn't fully fit, keeps the newest room samples (the tail — batches submit oldest-first, so unwritten is already chronological) and drops only the oldest excess, instead of discarding the entire backlog.
• Fixes an off-by-one where an exact-fit backlog (bufLen + len(unwritten) == capacity) was treated as over capacity and dropped in full; it now requeues in full.

Test coverage

Rewrote 4 pre-existing tests whose assertions baked in the old boundary bug, rather than just adding new ones:

• failed_retries_drop_when_over_capacity → failed_retries_keep_newest_when_over_capacity: was asserting full drop at a boundary that should now partially keep.
• no_backpressure_when_drop_on_overcapacity → kept_samples_never_exceed_capacity_after_partial_drop: retargeted at the invariant that actually matters post-fix (buffer never exceeds capacity), since "producer never blocks" isn't true in general once partial-keep can fill a buffer to exactly capacity.
• drops_failed_samples_when_requeue_would_meet_capacity → requeues_failed_samples_when_they_exactly_meet_capacity: exact-fit (bufLen + len(unwritten) == capacity) now requeues in full instead of dropping — that was the off-by-one in the old >= check.
• logs_and_counts_dropped_samples_when_over_capacity: updated to a genuine 3-samples-into-capacity-2 case so it exercises partial drop and asserts the new keptSamples log field.

Test plan

• go test ./internal/telemetry/... — 8/8 top-level tests pass, 0 failures (includes all 3 renamed subtests)
• go vet ./internal/telemetry/...
• gofmt -l clean
• golangci-lint run ./internal/telemetry/... — 0 issues

[ayushsingh82]

@elitegreg PR is up, ready for CI.